Remove the nightly CI jobs targeting the End-of-Life 2.x branch

[rsanchez15]

The 2.x branch has reached its End-of-Life, so it no longer needs to be built every night.
Removes the v2 job from the Ubuntu, Windows and system nightly workflows.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 32.29%. Comparing base (3a9a04b) to head (dafc601).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #552      +/-   ##
==========================================
- Coverage   33.87%   32.29%   -1.59%     
==========================================
  Files          16       16              
  Lines         614      576      -38     
  Branches      334      307      -27     
==========================================
- Hits          208      186      -22     
+ Misses        199      191       -8     
+ Partials      207      199       -8     

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[cferreiragonz]

LGTM