*: remove 19.x cluster versions and version gates

[dt]

[edit from @lucy-zhang] We're lagging behind on cleaning up cluster versions. We would like to have this test pass with only the last 2 major cluster versions permitted, as originally intended:

cockroach/pkg/clusterversion/keyed_versions.go

Lines 79 to 110 in 7a33017

	// Iterate through all versions known to be active. For example, if
	//
	// byRelease = ["2.0", "2.1", "19.1"]
	//
	// then we know that the current release cycle is 19.2, so mixed version
	// clusters are running at least 19.1, so anything slotted under 2.1 (like
	// 2.1-12) and 2.0 is always-on. To avoid interfering with backports, we're
	// a bit more lenient and allow one more release cycle until validation fails.
	// In the above example, we would tolerate 2.1-x but not 2.0-x.
	// Currently we're actually a few versions behind in enforcing a ban on old
	// versions/migrations. See #47447.
	if n := len(byRelease) - 5; n >= 0 {
	var buf strings.Builder
	for i, mami := range byRelease[:n+1] {
	s := "next release"
	if i+1 < len(byRelease)-1 {
	nextMM := byRelease[i+1]
	s = fmt.Sprintf("%d.%d", nextMM.major, nextMM.minor)
	}
	for _, nv := range mami.vs {
	fmt.Fprintf(&buf, "introduced in %s: %s\n", s, nv.Key)
	}
	}
	mostRecentRelease := byRelease[len(byRelease)-1]
	return errors.Errorf(
	"found versions that are always active because %d.%d is already "+
	"released; these should be removed:\n%s",
	mostRecentRelease.minor, mostRecentRelease.major,
	buf.String(),
	)
	}
	return nil

[thoszhang]

Here are all the obsolete version gates we need to clean up in 21.1:

• introduced in 19.2: Version19_1 @irfansharif pgwire: clean up old cluster versions passed to RegisterAuthMethod #56398
• introduced in 19.2: VersionStart19_2
• introduced in 19.2: VersionLearnerReplicas
• introduced in 19.2: VersionTopLevelForeignKeys
• introduced in 19.2: VersionAtomicChangeReplicasTrigger @tbg
• introduced in 19.2: VersionAtomicChangeReplicas @tbg
• introduced in 19.2: VersionTableDescModificationTimeFromMVCC
• introduced in 19.2: VersionPartitionedBackup @lucy-zhang
• introduced in 20.1: Version19_2 @andreimatei
• introduced in 20.1: VersionStart20_1 @itsbilal
• introduced in 20.1: VersionContainsEstimatesCounter @irfansharif kvserver: clean up VersionContainsEstimatesCounter migration from 20.1 #56401
• introduced in 20.1: VersionChangeReplicasDemotion @tbg
• introduced in 20.1: VersionSecondaryIndexColumnFamilies @solongordon
• introduced in 20.1: VersionNamespaceTableWithSchemas @solongordon
• introduced in 20.1: VersionProtectedTimestamps @ajwerner
• introduced in 20.1: VersionPrimaryKeyChanges @solongordon
• introduced in 20.1: VersionAuthLocalAndTrustRejectMethods @irfansharif pgwire: clean up old cluster versions passed to RegisterAuthMethod #56398
• introduced in 20.1: VersionPrimaryKeyColumnsOutOfFamilyZero @solongordon
• introduced in 20.1: VersionRootPassword
• introduced in 20.1: VersionNoExplicitForeignKeyIndexIDs @solongordon @lucy-zhang
• introduced in 20.1: VersionHashShardedIndexes @solongordon
• introduced in 20.1: VersionCreateRolePrivilege @solongordon
• introduced in 20.1: VersionStatementDiagnosticsSystemTables @andreimatei
• introduced in 20.1: VersionSchemaChangeJob @lucy-zhang
• introduced in 20.1: VersionSavepoints @knz
• introduced in 20.1: VersionTimeTZType @otan
• introduced in 20.1: VersionTimePrecision @otan

I've tagged people who may be able to help (with the help of git blame on where the version gates still exist), but feel free to reassign as necessary. Hopefully these are trivial changes, but it'd be good to have someone familiar with the surrounding logic actually do the cleanup.

[knz]

I'm ok doing those assigned to me but this would work somewhat better if my "package of work" was a separate issue