Rename 'Download' to 'Export'

[ChristophWurst]

Per #3993 (comment)

[codecov]

Codecov Report

Merging #4015 (d26af22) into main (57891f3) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main    #4015   +/-   ##
=========================================
  Coverage     29.45%   29.45%           
  Complexity      323      323           
=========================================
  Files           220      220           
  Lines          7577     7577           
  Branches       1002     1002           
=========================================
  Hits           2232     2232           
  Misses         5345     5345           

Flag	Coverage Δ
javascript	20.81% <ø> (ø)
php	67.40% <ø> (ø)

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Impacted Files	Coverage Δ
...ts/AppNavigation/CalendarList/CalendarListItem.vue	0.00% <ø> (ø)
...Navigation/CalendarList/PublicCalendarListItem.vue	0.00% <ø> (ø)
src/views/EditSidebar.vue	0.00% <ø> (ø)
src/views/EditSimple.vue	0.00% <ø> (ø)

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[tcitworld]

No better idea for the icon.

Needs a docs update too https://docs.nextcloud.com/server/latest/user_manual/en/groupware/calendar.html#edit-download-or-delete-a-calendar