prepare rules_go release 0.51.0-rc2 (#4183)

**What type of PR is this?**

Bug fix

**What does this PR do? Why is it needed?**

Deprecates the `RULES_GO_VERSION` constant in favor of `bazel_dep` and
keeps it at `0.50.0` for legacy consumers that may not be able to parse
RC suffixes.

**Which issues(s) does this PR fix?**

**Other notes for review**

go/def.bzl

@@ -123,9 +123,10 @@ _TOOLS_NOGO = [
 # new analyses may discover issues in existing builds.
 TOOLS_NOGO = [str(Label(l)) for l in _TOOLS_NOGO]
 
-# Current version or next version to be tagged. Gazelle and other tools may
-# check this to determine compatibility.
-RULES_GO_VERSION = "0.51.0-rc1"
+# Deprecated field previously used for version detection. This will not be
+# updated for new releases, use bazel_dep in MODULE.bazel to specify a minimum
+# version of rules_go instead.
+RULES_GO_VERSION = "0.50.0"
 
 go_context = _go_context
 gomock = _gomock

go/tools/releaser/prepare.go

@@ -40,10 +40,6 @@ with review. Specifically, prepare does the following:
 * Creates the release branch if it doesn't exist locally. Release branches
   have names like "release-X.Y" where X and Y are the major and minor version
   numbers.
-* Checks that RULES_GO_VERSION is set in go/def.bzl on the local release branch
-  for the minor version being released. RULES_GO_VERSION must be a sematic
-  version without the "v" prefix that Go uses, like "1.2.4". It must match
-  the -version flag, which does require the "v" prefix.
 * Creates an archive zip file from the tip of the local release branch.
 * Creates or updates a draft GitHub release with the given release notes.
   http_archive boilerplate is generated and appended to the release notes.
@@ -109,10 +105,9 @@ func runPrepare(ctx context.Context, stderr io.Writer, args []string) error {
 		return fmt.Errorf("release %s was already published", version)
 	}
 
-	// Check that RULES_GO_VERSION is set correctly on the release branch.
 	// If this is a minor release (x.y.0), create the release branch if it
 	// does not exist.
-	fmt.Fprintf(stderr, "checking RULES_GO_VERSION...\n")
+	fmt.Fprintf(stderr, "verifying release branch...\n")
 	rootDir, err := repoRoot()
 	if err != nil {
 		return err
@@ -125,19 +120,12 @@ func runPrepare(ctx context.Context, stderr io.Writer, args []string) error {
 	branchName := "release-" + majorMinor[len("v"):]
 	if !gitBranchExists(ctx, rootDir, branchName) {
 		if !isMinorRelease {
-			return fmt.Errorf("release branch %q does not exist locally. Fetch it, set RULES_GO_VERSION, add commits, and run this command again.", branchName)
-		}
-		if err := checkRulesGoVersion(ctx, rootDir, "HEAD", version); err != nil {
-			return err
+			return fmt.Errorf("release branch %q does not exist locally. Fetch it, add commits, and run this command again.", branchName)
 		}
 		fmt.Fprintf(stderr, "creating branch %s...\n", branchName)
 		if err := gitCreateBranch(ctx, rootDir, branchName, "HEAD"); err != nil {
 			return err
 		}
-	} else {
-		if err := checkRulesGoVersion(ctx, rootDir, branchName, version); err != nil {
-			return err
-		}
 	}
 
 	// Create an archive.
@@ -239,15 +227,3 @@ Release %s has been prepared and uploaded.
 
 	return nil
 }
-
-func checkRulesGoVersion(ctx context.Context, dir, refName, version string) error {
-	data, err := gitCatFile(ctx, dir, refName, "go/def.bzl")
-	if err != nil {
-		return err
-	}
-	rulesGoVersionStr := []byte(fmt.Sprintf(`RULES_GO_VERSION = "%s"`, version[len("v"):]))
-	if !bytes.Contains(data, rulesGoVersionStr) {
-		return fmt.Errorf("RULES_GO_VERSION was not set to %q in go/def.bzl. Set it, add commits, and run this command again.", version)
-	}
-	return nil
-}