Update protobuf requirement from ==5.29.* to ==6.30.* (#4938)

Updates the requirements on [protobuf](https://github.com/protocolbuffers/protobuf) to permit the latest version.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/main/protobuf_release.bzl)
- [Commits](https://github.com/protocolbuffers/protobuf/compare/v5.29.0-rc1...v6.30.1)

---
updated-dependencies:
- dependency-name: protobuf
  dependency-type: direct:production
...

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pyproject.toml

@@ -76,7 +76,7 @@ dependencies = [
   "phonenumberslite==8.13.*",
   "Pillow==11.1.*",
   "pretix-plugin-build",
-  "protobuf==5.29.*",
+  "protobuf==6.30.*",
   "psycopg2-binary",
   "pycountry",
   "pycparser==2.22",