generator: Consider removing @BetaApi from client header customization surface

[burkedavison]

1. Client stub settings

   • Example: https://github.com/googleapis/google-cloud-java/blob/main/java-iot/google-cloud-iot/src/main/java/com/google/cloud/iot/v1/stub/DeviceManagerStubSettings.java#L454-L471
   • Generation logic:

     sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/common/AbstractServiceStubSettingsClassComposer.java

     Line 374 in decd7f6

     .setType(FIXED_TYPESTORE.get("BetaApi"))

2. Client settings

   • Example: https://github.com/googleapis/google-cloud-java/blob/main/java-iot/google-cloud-iot/src/main/java/com/google/cloud/iot/v1/DeviceManagerSettings.java#L234-L237
   • Generation logic:

     sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/common/AbstractServiceSettingsClassComposer.java

     Line 436 in decd7f6

     .setType(FIXED_TYPESTORE.get("BetaApi"))

[diegomarquezp]

cc: @blakeli0
Unless there are plans to change the header customization surface in the future, this surface is otherwise stable and we should remove the annotation

Generator Classes

AbstractServiceSettings

How long has this API been around?
3 years

How much variation occured in the API over time?
The method generation code's last change was 3 years ago.

AbstractServiceStubSettings

How long has this API been around?
2+ years

How much variation occured in the API over time?
The generator method has not changed since its first implementation

Gax classes

ApiClientHeaderProvider

How long has this API been around?
6 years

How much variation occured in the API over time?
Last change was ~4 years ago