Bump third_party/nanopb/repo from f5f65a9 to a4bbd6a (#29652)

Bumps [third_party/nanopb/repo](https://github.com/nanopb/nanopb) from `f5f65a9` to `a4bbd6a`.
- [Commits](nanopb/nanopb@f5f65a9...a4bbd6a)

---
updated-dependencies:
- dependency-name: third_party/nanopb/repo
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>