storage: advance MinimumSupportedFormatVersion from 25.2 to 25.3

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

This commit advances storage's MinimumSupportedFormatVersion from
25.2 (pebble.FormatTableFormatV6) to 25.3 (pebble.FormatValueSeparation).

Epic: None
Release note: None

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

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

Author: celiala

pkg/storage/pebble.go

@@ -2447,7 +2447,7 @@ var pebbleFormatVersionMap = map[clusterversion.Key]pebble.FormatMajorVersion{
 // Cockroach code relies on unconditionally (like range keys). New stores are by
 // default created with this version. It should correspond to the minimum
 // supported binary version.
-const MinimumSupportedFormatVersion = pebble.FormatTableFormatV6
+const MinimumSupportedFormatVersion = pebble.FormatValueSeparation
 
 // pebbleFormatVersionKeys contains the keys in the map above, in descending order.
 var pebbleFormatVersionKeys = slices.SortedFunc(maps.Keys(pebbleFormatVersionMap), func(a, b clusterversion.Key) int {