Adds .gitea/workflows/ci.yml: on push to main and on pull requests, run
`go vet ./...` then `go test ./...` in a golang:1.21-alpine container.
Also removes .github/workflows/test.yml, which only triggered on a
`develop` branch that no longer exists and depended on an external
Discord notification action and an unset DISCORD_WEBHOOK secret.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
