clusterversion: remove Start21_1 (no longer applicable)

Fixes cockroachdb#65200. The last remaining 21.1 version (V21_1) can be removed as
part of cockroachdb#69828.

Release note: None

pkg/clusterversion/cockroach_versions.go

@@ -155,10 +155,8 @@ const (
 	_ Key = iota - 1 // want first named one to start at zero
 
 	// v21.1 versions.
-	//
-	// Start21_1 demarcates work towards CockroachDB v21.1.
-	Start21_1
 	// V21_1 is CockroachDB v21.1. It's used for all v21.1.x patch releases.
+	// TODO(irfansharif): This can be removed as part of once #69828 lands (bumping the min cluster version.
 	V21_1
 
 	// v21.1PLUS release. This is a special v21.1.x release with extra changes,
@@ -251,6 +249,7 @@ const (
 	// PebbleFormatVersioned ratchets Pebble's format major version to
 	// the version FormatVersioned.
 	PebbleFormatVersioned
+
 	// *************************************************
 	// Step (1): Add new versions here.
 	// Do not add new versions to a patch release.
@@ -275,18 +274,15 @@ const (
 // minor version until we are absolutely sure that no new migrations will need
 // to be added (i.e., when cutting the final release candidate).
 var versionsSingleton = keyedVersions{
-
-	// v21.1 versions. Internal versions defined here-on-forth must be even.
-	{
-		Key:     Start21_1,
-		Version: roachpb.Version{Major: 20, Minor: 2, Internal: 2},
-	},
+	// v21.1 versions.
 	{
 		// V21_1 is CockroachDB v21.1. It's used for all v21.1.x patch releases.
 		Key:     V21_1,
 		Version: roachpb.Version{Major: 21, Minor: 1},
 	},
 
+	// Internal versions must be even.
+
 	// v21.1PLUS version. This is a special v21.1.x release with extra changes,
 	// used internally for the 2021 Serverless offering.
 	//
@@ -436,7 +432,8 @@ var (
 	// binaryMinSupportedVersion is the earliest version of data supported by
 	// this binary. If this binary is started using a store marked with an older
 	// version than binaryMinSupportedVersion, then the binary will exit with
-	// an error.
+	// an error. This typically trails the current release by one (see top-level
+	// comment).
 	binaryMinSupportedVersion = ByKey(V21_1)
 
 	// binaryVersion is the version of this binary.

pkg/sql/set_cluster_setting.go

@@ -403,21 +403,6 @@ func runVersionUpgradeHook(
 	targetVersionStr := string(*value.(*tree.DString))
 	to.Version = roachpb.MustParseVersion(targetVersionStr)
 
-	start21_1 := clusterversion.ByKey(clusterversion.Start21_1)
-	if !params.extendedEvalCtx.Codec.ForSystemTenant() && from.Less(start21_1) {
-
-		// In the case that we're setting the cluster version to something that
-		// precedes the start of 21.1, which is permitted, if only because it's
-		// complex to prevent, then there's definitely no migrations to run and
-		// it may be hazardous due to assumptions about versions being even.
-		if to.Less(start21_1) {
-			return nil
-		}
-		// Otherwise, tell the migration layer that we're starting from the lowest
-		// allowable version.
-		from.Version = start21_1
-	}
-
 	// toSettingString already validated the input, and checked to
 	// see that we are allowed to transition. Let's call into our
 	// upgrade hook to run migrations, if any.