Update protobuf requirement from ==4.25.* to ==5.26.* (#3986)

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/v4.25.0-rc1...v5.26.0)

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

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pyproject.toml

@@ -77,7 +77,7 @@ dependencies = [
   "phonenumberslite==8.13.*",
   "Pillow==10.2.*",
   "pretix-plugin-build",
-  "protobuf==4.25.*",
+  "protobuf==5.26.*",
   "psycopg2-binary",
   "pycountry",
   "pycparser==2.21",