update GHA runners to ubuntu latest

[mmaeng]

Closes #107

github runner for ubuntu 18.04 was deprecated and GHA workflows were stuck. This updates workflows to use latest ubuntu runners.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (d989f2b) 98.56% compared to head (b3bd1d2) 98.56%.

❗ Current head b3bd1d2 differs from pull request most recent head 2ccbe03. Consider uploading reports for the commit 2ccbe03 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #108   +/-   ##
=======================================
  Coverage   98.56%   98.56%           
=======================================
  Files          14       14           
  Lines        1250     1250           
  Branches      209      209           
=======================================
  Hits         1232     1232           
  Misses         17       17           
  Partials        1        1           

Flag	Coverage Δ
unittests	98.56% <ø> (ø)

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