Enable caldav push notifications by default

[ChristophWurst]

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Is your feature request related to a problem? Please describe.

As a user of Nextcloud who doesn't use connected clients I want to see a web notification for calendar events.

Describe the solution you'd like

Revert #16970

Describe alternatives you've considered

#39158

Additional context

Needs admin upgrade docs because it will have a big influence on existing setups.

[ChristophWurst]

In a way this is "breaking" because systems will behave different after this has been deployed. Do we want this change to be included only in 28 or 27.1 as well @AndyScherzinger @jancborchardt?

[jancborchardt]

I’m fine with either, but I would say 27.1 to get it done?

[AndyScherzinger]

27.1 would also be fine with me 👍