CalVer: use 2 digits for the year and 2 digits for the month.

[bmarty]

Complement for #4187, following requirements from element-hq/element-meta#2664

This is how it will appear in the app:

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.31%. Comparing base (0b04e40) to head (e5aecbb).
Report is 39 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4192   +/-   ##
========================================
  Coverage    83.31%   83.31%           
========================================
  Files         1881     1881           
  Lines        49101    49101           
  Branches      5769     5769           
========================================
+ Hits         40909    40910    +1     
  Misses        6116     6116           
+ Partials      2076     2075    -1     

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