schemachanger: remove release_25_2 schema changer rules

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

This commit removes the frozen schema changer rules for release 25.2,
as they are no longer needed now that MinSupported has been bumped to
v25.3.

The release_25_2 rules directory contained version-specific dependency
rules and deprules test data (9,633 lines) that are obsolete after the
MinSupported bump.

Epic: None

Release note: None

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

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

Author: celiala

pkg/cli/testdata/declarative-rules/invalid_version

@@ -6,4 +6,3 @@ unsupported version number, the supported versions are:
  latest
  1000025.4
  1000025.3
- 1000025.2

pkg/sql/schemachanger/scplan/BUILD.bazel

@@ -17,7 +17,6 @@ go_library(
         "//pkg/sql/schemachanger/scplan/internal/opgen",
         "//pkg/sql/schemachanger/scplan/internal/rules",
         "//pkg/sql/schemachanger/scplan/internal/rules/current",
-        "//pkg/sql/schemachanger/scplan/internal/rules/release_25_2",
         "//pkg/sql/schemachanger/scplan/internal/rules/release_25_3",
         "//pkg/sql/schemachanger/scplan/internal/rules/release_25_4",
         "//pkg/sql/schemachanger/scplan/internal/scgraph",