fix(billing): BRP-17444 update entityId to global id (#843)

<!-- Ticket number or summary of work -->
# [BRP-17444]
https://bigcommercecloud.atlassian.net/browse/BRP-17444


## What changed?
<!-- Provide a bulleted list in the present tense -->
* updated mentions of `entityId` to the global `id`
* updated examples to show how the global id is supposed to be used

## Release notes draft
<!-- Provide an entry for the release notes using simple, conversational
language. Don't be too technical. Explain how the change will benefit
the merchant and link to the feature.

Examples:
* The newly-released [X feature] is now available to use. Now, you’ll be
able to [perform Y action].
* We're happy to announce [X feature], which can help you [perform Y
action].
* [X feature] helps you to create [Y response] using the [Z query
parameter]. Now, you can deliver [ex, localized shopping experiences for
your customers].
* Fixed a bug in the [X endpoint]. Now the [Y field] will appear when
you click [Z option]. -->
* Updated references of `entityId` to `id`. This matches the use of `id`
everywhere else on the platform

## Anything else?
<!-- Add related PRs, salient notes, additional ticket numbers, etc. -->

ping {names}


[BRP-17444]:
https://bigcommercecloud.atlassian.net/browse/BRP-17444?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

docs/integrations/apps/unified-billing/example-queries.mdx

@@ -28,12 +28,12 @@ To create a checkout, run the `createCheckout` mutation:
                       subscriptionId
                       status
                       product {
-                        entityId
+                        id
                         type
                         productLevel
                       }
                       scope {
-                        entityId
+                        id
                         type
                       }
                       pricingPlan {
@@ -67,12 +67,12 @@ To create a checkout, run the `createCheckout` mutation:
           "items": [
             {
               "product": {
-                "entityId": "1234",
+                "id": "bc/account/product/1234",
                 "type": "APPLICATION",
                 "productLevel": "Standard"
               },
               "scope": {
-                "entityId": "f0qyczpkb2",
+                "id": "bc/account/scope/f0qyczpkb2",
                 "type": "STORE"
               },
               "pricingPlan": {
@@ -111,12 +111,12 @@ To create a checkout, run the `createCheckout` mutation:
                         "subscriptionId": null,
                         "status": "PENDING",
                         "product": {
-                          "entityId": "1234",
+                          "id": "bc/account/product/1234",
                           "type": "APPLICATION",
                           "productLevel": "Standard"
                         },
                         "scope": {
-                          "entityId": "f0qyczpkb2",
+                          "id": "bc/account/scope/f0qyczpkb2",
                           "type": "STORE"
                         },
                         "pricingPlan": {
@@ -172,12 +172,12 @@ To fetch an existing checkout, copy and run the following query:
                 description
                 status
                 product {
-                  entityId
+                  id
                   productLevel
                   type
                 }
                 scope {
-                  entityId11
+                  id
                   type
                 }
                 pricingPlan {
@@ -218,12 +218,12 @@ To fetch an existing checkout, copy and run the following query:
                     "description": "Silver Tier Monthly Plan",
                     "status": "PENDING",
                     "product": {
-                      "entityId": "5",
+                      "id": "bc/account/product/5",
                       "productLevel": null,
                       "type": "APPLICATION"
                     },
                     "scope": {
-                      "entityId": "fsm6cvyxbj",
+                      "id": "bc/account/scope/fsm6cvyxbj",
                       "type": "STORE"
                     },
                     "pricingPlan": {
@@ -266,12 +266,12 @@ To fetch an existing checkout, copy and run the following query:
                     "description": "Gold Tier Annual Plan",
                     "status": "COMPLETE",
                     "product": {
-                      "entityId": "5",
+                      "id": "bc/account/product/5",
                       "productLevel": null,
                       "type": "APPLICATION"
                     },
                     "scope": {
-                      "entityId": "q6sxaezcnf",
+                      "id": "bc/account/scope/fsm6cvyxbj",
                       "type": "STORE"
                     },
                     "pricingPlan": {