logictest: clean up mixed-version tests for MinSupported bump

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

After bumping MinSupported from v25.2 to v25.3, tests that use the
cockroach-go-testserver predecessor binary configuration need cleanup:

Files with ONLY cockroach-go-testserver-25.2 are removed as they test
version-gating specific to a version below MinSupported:
- mixed_version_char: CHAR type version gating
- mixed_version_citext: CITEXT type version gating
- vector_index_mixed: Vector index version gating

Files with BOTH 25.2 and 25.3 testservers are updated to remove the
25.2 reference since it's below MinSupported:
- mixed_version_ltree: Tests ltree type availability after upgrade
- mixed_version_partial_stats: Tests partial statistics with WHERE clause

Also updates the M.4 runbook with guidance for handling test files:
- Guidance 1: Remove `onlyif config local-mixed-25.2` directives
- Guidance 2: Remove files with only old testserver, update files with multiple testservers

Part of #147634 (reference PR for this quarterly task).

Release note: None
Epic: None

Author: celiala

pkg/clusterversion/CLAUDE.md

@@ -1,4 +1,4 @@
-# LogicTest: cockroach-go-testserver-25.2 cockroach-go-testserver-25.3
+# LogicTest: cockroach-go-testserver-25.3
 
 # Sanity check that LTREE type is only allowed to be used once the cluster is
 # upgraded to 25.4.

pkg/sql/logictest/testdata/logic_test/mixed_version_char

@@ -1,4 +1,4 @@
-# LogicTest: cockroach-go-testserver-25.2 cockroach-go-testserver-25.3
+# LogicTest: cockroach-go-testserver-25.3
 
 # Sanity check that partial stats with a WHERE clause can only be collected once
 # the cluster is upgraded to 25.4.

pkg/sql/logictest/testdata/logic_test/mixed_version_citext

@@ -11,7 +11,7 @@ go_test(
         "//pkg/sql/logictest:testdata",  # keep
     ],
     exec_properties = {"test.Pool": "heavy"},
-    shard_count = 12,
+    shard_count = 7,
     tags = ["cpu:3"],
     deps = [
         "//pkg/base",