chore: put repo into legacylibrarian release-only mode

Attempting to regenerate using legacylibrarian will fail,
and releasing will not expect commits to be generated by
legacylibrarian.

Author: jskeet

.librarian/config.yaml

@@ -1,3 +1,11 @@
+# This repo is now in legacylibrarian "release-only mode"
+# as part of the migration to librarian.
+#
+# Attempting to regenerate using legacylibrarian will fail,
+# and releasing will not expect commits to be generated by
+# legacylibrarian.
+release_only_mode: true
+
 global_files_allowlist:
   # Allow the container to read and write the root `CHANGELOG.md`
   # file during the `release` step to update the latest client library