Merged PR branches should be cleaned up automatically everywhere, for a
consistent workflow across the estate.
- Flip default_delete_branch_after_merge from false to true on the 10 repos
that had it disabled.
- Add default_delete_branch_after_merge: true to the 3 repos that were missing
the field (terraform-git, terraform-provider-litellmvaultsecret,
vault-plugin-secrets-litellm).