build: Remove @internalapi from GAPIC Admin Base clients

Updates owlbot.py to stop injecting `@InternalApi` into `BaseBigtableTableAdminClient`, `BaseBigtableInstanceAdminClient`, and their respective Settings classes.

Since we recently exposed these Base classes to users via `getBaseClient()` as part of the GAPIC modernization effort, they are now part of the public API surface and should no longer be marked as internal. Stubs remain marked as internal.

Tracking Bug: b/475818901

Author: jinseopkim0

owlbot.py

@@ -32,14 +32,10 @@
 admin_internal_only = [
     'owl-bot-staging/v2/google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/admin/v2/stub/GrpcBigtableInstanceAdminCallableFactory.java',
     'owl-bot-staging/v2/google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/admin/v2/stub/GrpcBigtableInstanceAdminStub.java',
-    'owl-bot-staging/v2/google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/admin/v2/BaseBigtableInstanceAdminSettings.java',
     'owl-bot-staging/v2/google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/admin/v2/stub/GrpcBigtableTableAdminCallableFactory.java',
     'owl-bot-staging/v2/google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/admin/v2/stub/GrpcBigtableTableAdminStub.java',
-    'owl-bot-staging/v2/google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/admin/v2/BaseBigtableTableAdminSettings.java',
     'owl-bot-staging/v2/google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/admin/v2/stub/BigtableInstanceAdminStub.java',
     'owl-bot-staging/v2/google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/admin/v2/stub/BigtableTableAdminStub.java',
-    'owl-bot-staging/v2/google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/admin/v2/BaseBigtableInstanceAdminClient.java',
-    'owl-bot-staging/v2/google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/admin/v2/BaseBigtableTableAdminClient.java',
 ]
 
 # TODO: try to move the GAPIC surface hiding to the gapic-generator: