syncthing: fix HEAD build

[FiloSottile]

• Have you followed the guidelines for contributing?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
  • No, due to an unrelated upstream issue: request: add darwin/arm64 build for macOS ARM syncthing/syncthing#7128
• Does your build pass brew audit --strict <formula> (after doing brew install <formula>)?

---

The HEAD build is broken because the upstream branch changed to main, and because the binary expects the version number to match a certain pattern that starts with v\d+\.\d+\.\d+\.

[chenrui333]

Thanks @FiloSottile!

[BrewTestBot]

🤖 A scheduled task has triggered a merge.