relnotes: add 6.2.1 and 6.2.2

Welcome to the Mercurial Merge Request creation process:

• Set a simple title for your MR,
• All important information should be contained in your changesets' content or description,
• You can add some workflow-relevant information here (eg: when this depends on another MR),
• If your changes are not ready for review yet, click Start the title with Draft: under the title.

More details here:

• https://www.mercurial-scm.org/wiki/ContributingChanges
• https://www.mercurial-scm.org/wiki/Heptapod