Revert "chore: Update CODEOWNERS to change team ownership to cloud-sdk-librarian-admin (#2914)" (#2925)

As mentioned in
[comment](#2914 (comment)),
this is not how we want to handle codeowners in librarian. We want a
larger group to be able to approve PRs, but instead just route PRs to a
smaller team.

This reverts commit d5001e1.

Author: ldetmer

.github/CODEOWNERS

@@ -3,7 +3,7 @@
 # is the same as .gitignore.
 
 # The owners for all files in the repo.
-* @googleapis/cloud-sdk-librarian-admin
+* @googleapis/cloud-sdk-librarian-team
 
 /cmd/sidekick/ @googleapis/cloud-sdk-sidekick-team @googleapis/cloud-sdk-librarian-team
 /internal/sidekick/ @googleapis/cloud-sdk-sidekick-team @googleapis/cloud-sdk-librarian-team