mixedversion: remove special case for skipping current version

In the past, we used to bump CRDB's minSupportedVersion _before_
bumping CRDB's current version.

Consider the old version bump process:
  1. Bump minimum supported version (e.g. 25.2→25.3)
  2. Bump current version (e.g. 25.4→26.1)
  3. Bump minimum supported version again (e.g. 25.3→25.4)

After the first step, we would be in a state where the
mixed version framework would see v25.3 as a skippable
version (.1 and .3 ordinal versions are skippable), but
since the minimum supported version is 25.3, 25.2→25.4
is (temporarily) not a legal upgrade.

This required a special case to disallow skip upgrades
in the case of this edge case. However, we now bump the
current version _first_. This means we no longer need
to protect against the above and we actually hit a
different bug.

Since we now bump the current version first, the special
case will always view the current version as having multiple
supported predecessors and force a skip upgrade even if not
applicable.

Author: DarrylWong

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

@@ -79,7 +79,6 @@ import (
 	"sync"
 	"time"
 
-	"github.com/cockroachdb/cockroach/pkg/clusterversion"
 	"github.com/cockroachdb/cockroach/pkg/cmd/roachtest/cluster"
 	"github.com/cockroachdb/cockroach/pkg/cmd/roachtest/option"
 	"github.com/cockroachdb/cockroach/pkg/cmd/roachtest/registry"
@@ -94,7 +93,6 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/testutils/release"
 	"github.com/cockroachdb/cockroach/pkg/util/randutil"
 	"github.com/cockroachdb/errors"
-	"github.com/cockroachdb/version"
 )
 
 const (
@@ -613,14 +611,6 @@ func (t *Test) supportsSkipUpgradeTo(pred, v *clusterupgrade.Version) bool {
 	if t.options.minimumSupportedVersion.Series() == pred.Series() {
 		return false
 	}
-	// Special case for the current release series. This is useful to keep the
-	// test correct when we bump the minimum supported version separately from
-	// the current version.
-	r := clusterversion.Latest.ReleaseSeries()
-	currentMajor := version.MajorVersion{Year: int(r.Major), Ordinal: int(r.Minor)}
-	if currentMajor.Equals(v.Version.Major()) {
-		return len(clusterversion.SupportedPreviousReleases()) > 1
-	}
 
 	series := v.Version.Major()
 	switch {