Update UnifiedPush documentation as non-experimental

[PanderMusubi]

If UnifiedPush no longer is experimental, please update https://github.com/bitfireAT/davx5-manual/blob/main/settings.rst?plain=1#L48

[rfc2822]

Will be changed as soon as Push is enabled by default (bitfireAT/davx5-ose#1737), which is hopefully in the next version.