Draft: community/gcr4: upgrade to 4.3.0, move gcr-ssh-agent to gcr4, and many more cleanups for gcr3/gcr4

Fixes #15965. Began as a simple upgrade and make gcr-ssh-agent default job, and ended up growing into a lot more...

A rundown of all the changes in this MR:

• Upgrade gcr4 to 4.2.1
• Drop gcr-ssh-agent from gcr, move it to gcr4
• Drop dependency on gcr-ssh-agent from gcr and gcr4, move it into gnome package
• Add autostart .desktop file, wrapper and profile.d tweak to make gcr-ssh-agent the default when installed
• Move libgcr-ui-3.so.1 to gcr-base subpackage, update all packages to match