feat: require caldav backend

[st3iny]

Will be required after nextcloud/server#46510

• Wait for feat: add backend dependency type to info.xsd appstore#1414 (to pass validation)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 23.97%. Comparing base (6d1a23d) to head (5b3293f).
Report is 40 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6170      +/-   ##
============================================
- Coverage     23.97%   23.97%   -0.01%     
  Complexity      457      457              
============================================
  Files           247      247              
  Lines         11605    11606       +1     
  Branches       2177     2188      +11     
============================================
  Hits           2782     2782              
  Misses         8508     8508              
- Partials        315      316       +1     

Flag	Coverage Δ
javascript	15.46% <ø> (ø)
php	59.53% <ø> (-0.03%)	⬇️

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