Merge master to v4.0 (pingcap#14898)

session: make unit test TestOptimisticConflicts more stable pingcap#1…

…4746 (pingcap#14789)

store/helper: fill data in the information.tidb_hot_table for partiti…

…oned table (pingcap#14331)

ddl: check if the dropping index is primary key when doing "drop inde…

…x `primary` on...". (pingcap#14122) (pingcap#14426)

Merge release-3.0 to release-3.1 (pingcap#14361)

The following test cases are updated during the merge:
  - cmd/explaintest/r/explain_union_scan.result
  - planner/core/testdata/plan_normalized_suite_out.json

executor: add rollback in `releaseSysSession` (pingcap#14272)

ddl: check foreign key constraint when drop,modify,change column (pin…

…gcap#14043) (pingcap#14162)

Merge release-3.0 into release-3.1 (pingcap#13876)

store/tikv: fix lockTTL too large if local time is behind timestamp (p…

…ingcap#13868)

session: add more information for retry error (pingcap#13525) (pingca…

…p#13698)