fix: don't apply default calendar on Nextcloud < 29

[st3iny]

Fix https://github.com/nextcloud/calendar/issues/5942

The feature to set the default calendar is only available in Nextcloud >= 29 so users might be confused by the default calendar choice which they can't influence.

Simply use the old logic (first sorted calendar) on older instances.

[st3iny]

/backport to stable4.7

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 23.51%. Comparing base (71ce4bd) to head (b46ea0d). Report is 6 commits behind head on main.

Files	Patch %	Lines
src/store/calendarObjects.js	0.00%	3 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #5980 +/- ## ============================================ - Coverage 23.52% 23.51% -0.01% Complexity 455 455 ============================================ Files 246 246 Lines 11716 11718 +2 Branches 2144 2155 +11 ============================================ Hits 2756 2756 Misses 8646 8646 - Partials 314 316 +2 ``` | [Flag](https://app.codecov.io/gh/nextcloud/calendar/pull/5980/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud) | Coverage Δ | | |---|---|---| | [javascript](https://app.codecov.io/gh/nextcloud/calendar/pull/5980/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud) | `15.16% <0.00%> (-0.01%)` | :arrow_down: | | [php](https://app.codecov.io/gh/nextcloud/calendar/pull/5980/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud) | `58.46% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud#carryforward-flags-in-the-pull-request-comment) to find out more.

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