v0.3.1 — the retired Go Report Card badge is gone

Go Report Card was sunset after ten years of grading Go. Its endpoint now
serves a badge that reads "go report: retired" and the report link lands on
a farewell page, so the README advertised a grade nobody can look up.

The checks it stood for run closer to home: gofmt, vet, staticcheck and
govulncheck are all in make check, on every commit. License, Releases and Go Reference stay.

README only. No code changed, so the API is the one v0.3.0 shipped.