cleanup!: deprecate public bigtable bazel targets (#8581)
cleanup!: deprecate public bigtable bazel targets (#8581)

Part of #3701

Marks all Bigtable BUILD targets as private by default. cc_test targets
that were previously public are now private. The
//google/cloud/bigtable:bigtable_client_testing will remain public.


googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp