analysis/lint: accept new Go versions

Go versions in go.mod files can now look like "go1.21rc5" or "go1.21.0".
See https://go.dev/doc/toolchain#versions.

Closes: gh-1431
(cherry picked from commit ac367e4)

analysis/lint/lint.go

@@ -8,6 +8,7 @@ import (
 	"go/ast"
 	"go/build"
 	"go/token"
+	"regexp"
 	"strconv"
 	"strings"
 
@@ -206,21 +207,28 @@ func (v *VersionFlag) String() string {
 	return fmt.Sprintf("1.%d", *v)
 }
 
-func (v *VersionFlag) Set(s string) error {
-	if len(s) < 3 {
-		return fmt.Errorf("invalid Go version: %q", s)
-	}
-	if s[0] != '1' {
-		return fmt.Errorf("invalid Go version: %q", s)
-	}
-	if s[1] != '.' {
-		return fmt.Errorf("invalid Go version: %q", s)
+var goVersionRE = regexp.MustCompile(`^(?:go)?1.(\d+).*$`)
+
+// ParseGoVersion parses Go versions of the form 1.M, 1.M.N, or 1.M.NrcR, with an optional "go" prefix. It assumes that
+// versions have already been verified and are valid.
+func ParseGoVersion(s string) (int, bool) {
+	m := goVersionRE.FindStringSubmatch(s)
+	if m == nil {
+		return 0, false
 	}
-	i, err := strconv.Atoi(s[2:])
+	n, err := strconv.Atoi(m[1])
 	if err != nil {
+		return 0, false
+	}
+	return n, true
+}
+
+func (v *VersionFlag) Set(s string) error {
+	n, ok := ParseGoVersion(s)
+	if !ok {
 		return fmt.Errorf("invalid Go version: %q", s)
 	}
-	*v = VersionFlag(i)
+	*v = VersionFlag(n)
 	return nil
 }