Update goreleaser to publish manifest file via hc-releases

The recent goreleaser v1.3.0 release allows for custom publishers to declare `extra_files` configuration. This will allow us to upload the manifest file to releases.hashicorp.com, which should be ingested by the Terraform Registry to properly set protocol versions, rather than being reliant on the legacy and HashiCorp-only x-terraform-protocol-version(s) headers.

This also renames the manifest file slightly to match the documented convention at https://www.terraform.io/registry/providers/publishing#terraform-registry-manifest-file

```console
$ goreleaser --version
goreleaser version 1.3.0
commit: 3d2042cbf4d3b08a8395f0a4099e4bd31520dc24
built by: homebrew
goos: darwin
goarch: amd64

https://goreleaser.com
$ goreleaser check
   • loading config file       file=.goreleaser.yml
   • checking config:
   • config is valid
```

.goreleaser.yml

@@ -28,13 +28,16 @@ builds:
     mod_timestamp: '{{ .CommitTimestamp }}'
 checksum:
   extra_files:
-    - glob: '{{ .ProjectName }}_manifest.json'
+    - glob: 'terraform-registry-manifest.json'
       name_template: '{{ .ProjectName }}_{{ .Version }}_manifest.json'
   name_template: '{{ .ProjectName }}_{{ .Version }}_SHA256SUMS'
   algorithm: sha256
 publishers:
   - name: hc-releases
     checksum: true
+    extra_files:
+      - glob: 'terraform-registry-manifest.json'
+        name_template: '{{ .ProjectName }}_{{ .Version }}_manifest.json'
     signature: true
     cmd: hc-releases upload-file {{ abs .ArtifactPath }}
     env:
@@ -43,7 +46,7 @@ publishers:
       - AWS_SESSION_TOKEN={{ .Env.AWS_SESSION_TOKEN }}
 release:
   extra_files:
-    - glob: '{{ .ProjectName }}_manifest.json'
+    - glob: 'terraform-registry-manifest.json'
       name_template: '{{ .ProjectName }}_{{ .Version }}_manifest.json'
   ids:
     - none