chore: clarify that barretenberg mirror should not take PRs (#3303)

We can't disable PRs there unfortunately. Next best thing. We should
revisit the mirror, but for now it has new bb issues.

barretenberg/.github/pull_request_template.md

@@ -1,16 +1,6 @@
-# Description
-
-Please provide a paragraph or two giving a summary of the change, including relevant motivation and context.
-
-# Checklist:
-
-- [ ] I have reviewed my diff in github, line by line.
-- [ ] Every change is related to the PR description.
-- [ ] The branch has been merged with/rebased against the head of its merge target.
-- [ ] There are no unexpected formatting changes, superfluous debug logs, or commented-out code.
-- [ ] There are no circuit changes, OR a cryptographer has been assigned for review.
-- [ ] New functions, classes, etc. have been documented according to the doxygen comment format. Classes and structs must have `@brief` describing the intended functionality.
-- [ ] If existing code has been modified, such documentation has been added or updated.
-- [ ] No superfluous `include` directives have been added.
-- [ ] I have [linked](https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue) to any issue(s) it resolves.
-- [ ] I'm happy for the PR to be merged at the reviewer's next convenience.
+:warning: :warning: :warning: DO NOT CREATE THIS PR. :warning: :warning: :warning:
+You are creating a PR into the wrong repository.
+Please migrate your PR to https://github.com/AztecProtocol/aztec-packages/pulls with `scripts/migrate_barretenberg_branch.sh` in the aztec-packages repo.
+```
+Usage: scripts/migrate_barretenberg_branch.sh <branch> <commit_message>
+```