Deprecate welcome bot welcome_user_id support

Signed-off-by: Michael Telatynski <[email protected]>

docs/config.md

@@ -137,7 +137,7 @@ complete re-branding/private labeling, a more personalised experience can be ach
    This setting is ignored if your homeserver provides `/.well-known/matrix/client` in its well-known location, and the JSON file
    at that location has a key `m.tile_server` (or the unstable version `org.matrix.msc3488.tile_server`). In this case, the
    configuration found in the well-known location is used instead.
-10. `welcome_user_id`: An optional user ID to start a DM with after creating an account. Defaults to nothing (no DM created).
+10. `welcome_user_id`: **DEPRECATED** An optional user ID to start a DM with after creating an account. Defaults to nothing (no DM created).
 11. `custom_translations_url`: An optional URL to allow overriding of translatable strings. The JSON file must be in a format of
     `{"affected|translation|key": {"languageCode": "new string"}}`. See https://github.com/matrix-org/matrix-react-sdk/pull/7886 for details.
 12. `branding`: Options for configuring various assets used within the app. Described in more detail down below.