Mark background jobs as time sensitive/insensitive

[ChristophWurst]

This contributes the Calendar part of nextcloud/groupware#28.

Only the hourly sync job is time sensitive, the rest can be done at off times.

method_exists checks will allow us to ship this to older Nextcloud releases.

[codecov]

Codecov Report

Merging #2645 (d0fff89) into main (10aa075) will decrease coverage by 0.29%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##               main    #2645      +/-   ##
============================================
- Coverage     69.25%   68.95%   -0.30%     
- Complexity      247      248       +1     
============================================
  Files            21       21              
  Lines           696      699       +3     
============================================
  Hits            482      482              
- Misses          214      217       +3     

Impacted Files	Coverage Δ
lib/Cron/SocialUpdateRegistration.php	0.00% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 10aa075...d0fff89. Read the comment docs.