clusterversion: advance MinSupported to 25.3

Part of the quarterly M.4 "Bump MinSupported" task as outlined in
`pkg/clusterversion/README.md`.

This commit advances MinSupported to 25.3.

Older gates will be prefixed with TODO_Delete_ in a followup commit/PR.

Epic: None
Release note: None

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: celiala

pkg/clusterversion/cockroach_versions.go

@@ -322,7 +322,7 @@ var versionTable = [numKeys]roachpb.Version{
 const Latest Key = numKeys - 1
 
 // MinSupported is the minimum logical cluster version supported by this branch.
-const MinSupported Key = V25_2
+const MinSupported Key = V25_3
 
 // PreviousRelease is the logical cluster version of the previous release (which must
 // have at least an RC build published).