Break free from Knative actions

[imjasonh]

Largely copy/pasted from ko-build/ko#364

• Rename knative-*.yaml to drop the knative- prefix
• Remove release-related checks; we don't need them
• Remove Knative copyright headers
• Remove unnecessary steps/checks that either don't apply to this repo, or are always true for this repo
• Remove unnecessary matrixes that change the name of checks
• Run workflows on main or PRs to main (not master or release-*)
• Update Go version to 1.16.x everywhere

[dprotaso]

🎉