Bump third_party/nanopb/repo from a4bbd6a to 9766c45 (#29902)

Bumps [third_party/nanopb/repo](https://github.com/nanopb/nanopb) from `a4bbd6a` to `9766c45`.
- [Commits](nanopb/nanopb@a4bbd6a...9766c45)

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

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