Rename stub.%l to grader.%l in Communication tasks.

[veluca93]

Fixes #1705.

[codecov]

Codecov Report

❌ Patch coverage is 85.03937% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.16%. Comparing base (dfa0b66) to head (8c13aa8).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
cmscontrib/loaders/italy_yaml.py	0.00%	8 Missing ⚠️
cmscontrib/loaders/tps.py	0.00%	8 Missing ⚠️
cms/grading/tasktypes/Communication.py	85.71%	2 Missing ⚠️
cmstestsuite/unit_tests/cmscontrib/Update49Test.py	96.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1711      +/-   ##
==========================================
+ Coverage   55.04%   55.16%   +0.11%     
==========================================
  Files         339      341       +2     
  Lines       27401    27491      +90     
==========================================
+ Hits        15083    15165      +82     
- Misses      12318    12326       +8     

Flag	Coverage Δ
functionaltests	0.00% <0.00%> (ø)
unittests	55.16% <85.03%> (+0.11%)	⬆️

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