remove ubuntu 18.04

[lidaobing]

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[codecov]

Codecov Report

Patch coverage: 57.14% and project coverage change: +0.70% 🎉

Comparison is base (30b3c59) 44.54% compared to head (00356f9) 45.25%.
Report is 13 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #523      +/-   ##
==========================================
+ Coverage   44.54%   45.25%   +0.70%     
==========================================
  Files          68       61       -7     
  Lines        7736     7549     -187     
==========================================
- Hits         3446     3416      -30     
+ Misses       4290     4133     -157     

Files Changed	Coverage Δ
src/iptux/DialogGroup.cpp	41.90% <33.33%> (+3.12%)	⬆️
src/iptux/Application.cpp	41.14% <55.55%> (-0.81%)	⬇️
src/iptux/DataSettings.cpp	51.86% <100.00%> (-0.12%)	⬇️
src/iptux/MainWindow.cpp	27.16% <100.00%> (+0.38%)	⬆️

... and 27 files with indirect coverage changes

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