Mark gflags as no longer updated (#13191)

Author: kbaikov

stubs/python-gflags/METADATA.toml

@@ -1,2 +1,9 @@
 version = "3.1.*"
 upstream_repository = "https://github.com/google/python-gflags"
+no_longer_updated = true
+extra_description = """\
+`python-gflags` has been merged into
+[Abseil Python Common Libraries](https://github.com/abseil/abseil-py).
+
+Please see [the guidelines](absl_migration/migration_guidelines.md)
+for migrating to [Abseil](https://github.com/abseil/abseil-py)."""