Since there is no go.mod, CI has to create one on the fly (using
"go mod init" and "go get"). In addition, "make docs" no longer works
with Go 1.23+, since "go run ./.tool/version-doc.go" requires a module.
Add go.mod (with no dependencies, and the go directive set to the
minimal Go version we support and test with), and remove the go.mod
creation from CI.
Since schema/validate is now a separate module, run golangci-lint
for it separately. To avoid using golangci-lint-action twice, bump it
to v9 and use its install-only mode, running golangci-lint directly.
Fixes#1280.
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
