Bump actions/setup-go from 3 to 4

Bumps [actions/setup-go](https://github.com/actions/setup-go) from 3 to 4.
- [Release notes](https://github.com/actions/setup-go/releases)
- [Commits](actions/setup-go@v3...v4)

---
updated-dependencies:
- dependency-name: actions/setup-go
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Document FindFilesExcept and FindFirstFile in README

Pass repo-relative paths to gitignore matcher

Update README