[MS2] Revamp release process #2

[ppkarwasz]

After completing #1777, we can proceed with a practical application of the changes we introduces by:

• Decrease Log4j testing time #1778,
• Use release process on the primary repository #1779,
• preparing an actual release (e.g. 2.21.0), possibly performed by a PMC member that hasn't done any releases for a long time.

[ppkarwasz]

The 2.21.0 release was prepared using the new release process: https://lists.apache.org/thread/60cxs4894lqynvjn00lwgbl51c5mk5f1