[Backport release-24.05] itsycal: rfc format; itsycal: add meta.changelog; ityscal: 0.15.3 -> 0.15.4

[Backport release-24.05] itsycal: rfc format; itsycal: add meta.changelog; ityscal: 0.15.3 -> 0.15.4 #854648