mixedversion: fix skip upgrade minSupportedVersion edge case

This change adds a check before skipping upgrades that doing
so will still generate at least one upgrade that can allow
user hooks to run. This happens if the minimum supported
version is one skippable version behind the current version.

Author: DarrylWong

pkg/cmd/roachtest/roachtestutil/mixedversion/mixedversion.go

@@ -1069,6 +1069,20 @@ func (t *Test) chooseUpgradePath() ([]*clusterupgrade.Version, error) {
 			return []*clusterupgrade.Version{pred}, nil
 		}
 
+		// If the predecessor same series as the minimum supported version,
+		// we need to check that it isn't the only possible upgrade where
+		// user steps can run. If it is, don't allow a skip upgrade or else
+		// we will generate a plan with zero upgrades that are able to run user hooks.
+		if pred.Series() == t.options.minimumSupportedVersion.Series() {
+			maxTestUpgrades, err := release.MajorReleasesBetween(&t.options.minimumSupportedVersion.Version, &currentVersion.Version)
+			if err != nil {
+				return nil, err
+			}
+			if maxTestUpgrades == 1 {
+				return []*clusterupgrade.Version{pred}, nil
+			}
+		}
+
 		predPred, err := t.options.predecessorFunc(t.prng, pred, t.options.minimumSupportedVersion, t.options.minimumBootstrapVersion)
 		if err != nil {
 			return nil, err
@@ -1594,8 +1608,9 @@ func assertValidTest(test *Test, fatalFunc func(...interface{})) {
 
 	currentVersion := clusterupgrade.CurrentVersion()
 	msv := test.options.minimumSupportedVersion
-	// The minimum supported version should be strictly less than the current version
-	validVersion := currentVersion.AtLeast(msv) && !currentVersion.Equal(msv)
+	// The current version should be at least one release series newer from
+	// the minimum supported version.
+	validVersion := currentVersion.CompareSeries(msv.Version) == 1
 
 	if !validVersion {
 		fail(