Add a background task to fetch qgis versions

[dimasciput]

Implement #296

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.90 🎉

Comparison is base (0a86431) 76.83% compared to head (5868679) 77.73%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #297      +/-   ##
==========================================
+ Coverage   76.83%   77.73%   +0.90%     
==========================================
  Files         128      132       +4     
  Lines        4813     4937     +124     
==========================================
+ Hits         3698     3838     +140     
+ Misses       1115     1099      -16     

Impacted Files	Coverage Δ
qgis-app/plugins/tasks/update_qgis_versions.py	100.00% <100.00%> (ø)
qgis-app/plugins/tests/test_task.py	100.00% <100.00%> (ø)
qgis-app/plugins/tests/test_utils.py	100.00% <100.00%> (ø)
qgis-app/plugins/utils.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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