main/postmarketos-release-upgrade: upgrade to 1.6.0

https://gitlab.com/postmarketOS/postmarketos-release-upgrade/-/compare/1.4.0...1.6.0

Release 1.5.0

Features:
* Add v23.12
* CI: add v23.06 -> v23.12
* Check /boot partition size
* Check for installed mrtest packages

Other:
* CI: Drop v22.12 check (EOL)

Release 1.6.0

Features:
* Check for hardcoded versions