roachtest: update version map and fixtures

This commit adds the recently release 20.2.8 to the version map.
And generates test fixtures.

Release note: None

pkg/cmd/roachtest/test_runner.go

@@ -1197,7 +1197,7 @@ func PredecessorVersion(buildVersion version.Version) (string, error) {
 	// (see runVersionUpgrade). The same is true for adding a new key to this
 	// map.
 	verMap := map[string]string{
-		"21.1": "20.2.6",
+		"21.1": "20.2.8",
 		"20.2": "20.1.15",
 		"20.1": "19.2.11",
 		"19.2": "19.1.11",

pkg/cmd/roachtest/versionupgrade.go

@@ -94,7 +94,7 @@ func runVersionUpgrade(ctx context.Context, t *test, c *cluster, buildVersion ve
 		// The version to create/update the fixture for. Must be released (i.e.
 		// can download it from the homepage); if that is not the case use the
 		// empty string which uses the local cockroach binary.
-		newV := "20.1.15"
+		newV := "20.2.8"
 		predV, err := PredecessorVersion(*version.MustParse("v" + newV))
 		if err != nil {
 			t.Fatal(err)