Write Qubes OS 4.3 release notes

[marmarek]

Similarly to other releases, create a page listing major changes since previous release.

The content can be built based on issues with "targets-4.3" label, possibly combined also with "release notes" label. But it's probably worth reviewing issues without release notes label too (and add it when relevant?) as it isn't applied consistently :(

[alimirjamali]

Some notes:

The old off-line release notes was simplified during QubesOS/qubes-qubes-release@09ff941

Also related:
#9757

[jmynes]

Perhaps something like Release Please with Conventional Commits would be useful for generating a changelog for each future release?
https://github.com/googleapis/release-please