Drop compiler running on a deprecated (by GH actions) version of Ubuntu

[offa]

Clang 9 and 10 use an Ubuntu 18.04 based image, which isn't supported by the latest version of the checkout action.

https://github.com/actions/checkout/issues/1442

[codecov[bot]]

Codecov Report

Merging #194 (5a99e65) into master (7155ad2) will not change coverage. Report is 2 commits behind head on master. The diff coverage is 66.66%.

@@           Coverage Diff           @@
##           master     #194   +/-   ##
=======================================
  Coverage   34.84%   34.84%           
=======================================
  Files          54       54           
  Lines        2330     2330           
  Branches      355      355           
=======================================
  Hits          812      812           
  Misses       1429     1429           
  Partials       89       89           

Files Changed	Coverage Δ
src/main/c/seasocks/StreamingResponse.cpp	0.00% <0.00%> (ø)
src/main/c/HybiPacketDecoder.cpp	73.43% <100.00%> (ø)

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