server: update prev logic to account for more releases per year

Before 2024, we shipped 2 releases a year.
In 2024, we shipped 3 releases. And 2025 onwards, we will
be shipping 4 releases a year.

This PR adjusts prev() to account for the above shift
to more releases per year.

Release note: None
Epic: REL-1751
Release justification: test-only fix, to unblock release-process.

Author: celiala

pkg/server/version_cluster_test.go

@@ -134,8 +134,20 @@ func prev(version roachpb.Version) roachpb.Version {
 		if version.Minor > 1 {
 			return roachpb.Version{Major: version.Major, Minor: version.Minor - 1}
 		}
-		// Here we assume that there's going to only be 2 releases per year.
-		// Otherwise we'd need to keep some history of what releases we've had.
+
+		// version is the first release of that year, e.g. MM.1
+
+		v25_1 := roachpb.Version{Major: 25, Minor: 1}
+		if v25_1.Equal(version) {
+			// For 2024, we had 3 releases that year.
+			return roachpb.Version{Major: 24, Minor: 3}
+		}
+		if v25_1.Less(version) {
+			// 2025 onwards, we had 4 releases per year.
+			return roachpb.Version{Major: version.Major - 1, Minor: 4}
+		}
+
+		// Prior to 2024, we had 2 releases per year.
 		return roachpb.Version{Major: version.Major - 1, Minor: 2}
 	}
 
@@ -157,6 +169,50 @@ func prev(version roachpb.Version) roachpb.Version {
 	}
 }
 
+func TestPrev(t *testing.T) {
+	defer leaktest.AfterTest(t)()
+	defer log.Scope(t).Close(t)
+
+	tests := []struct {
+		this, prev roachpb.Version
+	}{
+		// releases 2025+: 4 releases per year.
+		{
+			this: roachpb.Version{Major: 28, Minor: 4},
+			prev: roachpb.Version{Major: 28, Minor: 3},
+		},
+		{
+			this: roachpb.Version{Major: 28, Minor: 1},
+			prev: roachpb.Version{Major: 27, Minor: 4},
+		},
+		{
+			this: roachpb.Version{Major: 26, Minor: 1},
+			prev: roachpb.Version{Major: 25, Minor: 4},
+		},
+		// releases 2024: 3 releases per year.
+		{
+			this: roachpb.Version{Major: 25, Minor: 1},
+			prev: roachpb.Version{Major: 24, Minor: 3},
+		},
+		// releases 2019-2023: Calendar versioning, with 2 releases per year
+		{
+			this: roachpb.Version{Major: 24, Minor: 1},
+			prev: roachpb.Version{Major: 23, Minor: 2},
+		},
+		{
+			this: roachpb.Version{Major: 21, Minor: 1},
+			prev: roachpb.Version{Major: 20, Minor: 2},
+		},
+	}
+	for _, test := range tests {
+		t.Run("", func(t *testing.T) {
+			if p := prev(test.this); p != test.prev {
+				t.Errorf("expected %s, got %s", test.prev, p)
+			}
+		})
+	}
+}
+
 func TestClusterVersionPersistedOnJoin(t *testing.T) {
 	defer leaktest.AfterTest(t)()
 	defer log.Scope(t).Close(t)