Explicitly select PHP 8.0 in CI

[phil-davis]

No description provided.

[codecov]

Codecov Report

Merging #19 (8781a45) into master (1061edb) will decrease coverage by 0.57%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master      #19      +/-   ##
============================================
- Coverage     96.87%   96.29%   -0.58%     
  Complexity        3        3              
============================================
  Files             1        1              
  Lines            32       27       -5     
============================================
- Hits             31       26       -5     
  Misses            1        1              

Impacted Files	Coverage Δ	Complexity Δ
lib/Service.php	96.29% <0.00%> (-0.58%)	3.00% <0.00%> (ø%)

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