Release 1.6.0

[csuzhangxc]

This is an issue tracking the tidb-operator v1.6.0 release.

Previous:

• 5619

also ref #5108, #5590

Pending PRs:

TODO:

• [x] Update default version of TiDB Operator #5641
• [x] Update TiDB version and tools version if necessary #5641
• [x] Update the TiDB version and tools version in docs-tidb-operator repo https://github.com/pingcap/docs-tidb-operator/pull/2568
• [x] Update release notes https://github.com/pingcap/docs-tidb-operator/pull/2567
  • Add what's new (v1.x.0 GA only)
• [x] Update Github Releases
• [x] Build and publish docker images/helm charts
• [x] Announcement (email, etc.)
• [x] Update TiDB Operator version in docs-tidb-operator repo (both for dev and stable branch) https://github.com/pingcap/docs-tidb-operator/pull/2569