Add 11.2.1 release notes

[radarhere]

Suggestion for #8722 (comment)

[hugovk]

Thanks, I've updated this to:

• remove the release notes for the incomplete 11.2.0, as it never completed, it never happened
• move the things that would have been released in 11.2.0 into 11.2.1
• note in 11.2.1 that AVIF is supported but needs to be built from source
• mention at the top of the 11.2.1 release notes the reason why 11.2.0 doesn't exist