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

main/postmarketos-release-upgrade/APKBUILD

 # Maintainer: Oliver Smith <ollieparaoid@postmarketos.org>
 pkgname=postmarketos-release-upgrade
-pkgver=1.4.0
+pkgver=1.6.0
 pkgrel=0
 pkgdesc="Upgrade from one pmOS release to the next one"
 url="https://gitlab.com/postmarketOS/postmarketos-release-upgrade/"
@@ -16,5 +16,5 @@ package() {
 }
 
 sha512sums="
-2b0515f0b4261b1ea51b024db44ce390e1a5688f30b6759703e94cab528665f2a4d90dc977fe4072eda90cc3529738534468e2117965d74120d8515c7283df18  postmarketos-release-upgrade-1.4.0.tar.bz2
+c7cf29854821d9f332bed702ab8dbd4424043e1b61f99328e0987f446fb517899db28c7fd8bb00f2f6a9012164896678f91e9ef0ba41da4f92f03f9b85ac0202  postmarketos-release-upgrade-1.6.0.tar.bz2
 "