doc: add the change log for release 2.1.1 (pingcap#8666)

CHANGELOG: add changelog for 2.1.0-ga (pingcap#8526)

CHANGELOG: Add changelog for 2.0.9 (pingcap#8354)

add changelog for v2.1.0-rc.5 (pingcap#8272) (pingcap#8275)

add changelog for 2.1.0 rc4 (pingcap#8020) (pingcap#8027)

CHANGELOG: Add 2.0.8 change logs (pingcap#7908)

domain: fast new a etcd session when the session is stale in the sche…

…maVersionSyncer (pingcap#7774)

CHANGELOGS: Add change logs for 2.1.0-rc.2 (pingcap#7705)

CHANGELOG: Add 2.0.7 change logs (pingcap#7641)

plan: fix a bug when using correlated column as index (pingcap#7357)