all: drop support for Go 1.17

Now that we've released v0.6.0, that will be the last feature release to
feature support for Go 1.17. The upcoming v0.7.0 will be Go 1.18+.

Code-wise, the cleanup here isn't super noticeable,
but it will be easier to work on features like VCS-aware version
information and generics support without worrying about Go 1.17.
Plus, now CI is back to being much faster.

Note how "go 1.18" in go.mod makes "go mod tidy" more aggressive.

position.go

@@ -15,6 +15,8 @@ import (
 )
 
 func isDirective(text string) bool {
+	// TODO: can we remove the check for "// +build" now that we require Go 1.18
+	// or later? we should update the tests too.
 	return strings.HasPrefix(text, "//go:") || strings.HasPrefix(text, "// +build")
 }