Post-branch updates to release-related documentation:

• Bump copyright years.
• Note in parenthesis 11.3 may be the last 11.x release.
• Prune stale errata items from 11.2-RELEASE.
• Fix mailing list in the readme page.
• Bump version numbers accordingly.

This has no effect or impact on the in-progress 11.3-RC1
build.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation