hard code protobuf version while we're on v2

[henderkes]

What does this PR do?

Checklist before merging

If your PR involves the changes mentioned below and completed the action, please tick the corresponding option.
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• If you modified *.php or *.json, run them locally to ensure your changes are valid:
  • composer cs-fix
  • composer analyse
  • composer test
  • bin/spc dev:sort-config
• If it's an extension or dependency update, please ensure the following:
  • Add your test combination to src/globals/test-extensions.php.
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