translate: add notice to recommend v3 gapic client for new projects
translate: add notice to recommend v3 gapic client for new projects

Change-Id: I528f5ac0242039d8216653a98ffbc25204053f20
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/47472
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go