feat(bigtable): update legacy escape hatch to return V2 admin clients

[jinseopkim0]

This PR updates the legacy BigtableTableAdminClient and BigtableInstanceAdminClient escape hatches (getBaseClient()) to return the modern V2 admin clients (BigtableTableAdminClientV2 and BigtableInstanceAdminClientV2) instead of the autogenerated base clients (BaseBigtableTableAdminClient and BaseBigtableInstanceAdminClient).

[gemini-code-assist]

Code Review

This pull request transitions BigtableInstanceAdminClient and BigtableTableAdminClient to use the V2 client classes (BigtableInstanceAdminClientV2 and BigtableTableAdminClientV2) instead of the base autogenerated clients. Key feedback includes resolving a compilation error in BigtableTableAdminClientV2.createAwaitConsistencyCallable due to a type mismatch with EnhancedBigtableTableAdminStub, and updating outdated Javadocs for getBaseClient() in both admin clients to accurately reflect the new V2 return types.

[jinseopkim0]

/gemini review

[gemini-code-assist]

Code Review

This pull request refactors BigtableInstanceAdminClient and BigtableTableAdminClient to use the newer V2 client implementations instead of the older Base clients. The review comments correctly identify compilation errors in BigtableTableAdminClient where EnhancedBigtableTableAdminStub is passed to methods expecting GrpcBigtableTableAdminStub. To fix this, the reviewer suggests exposing the underlying GrpcBigtableTableAdminStub from EnhancedBigtableTableAdminStub and using it during the client initialization.