Add PHP 8.0 to text matrix

[SteelWagstaff]

Partial fix for https://github.com/pressbooks/private/issues/801

[codecov]

Codecov Report

Merging #956 (a4c7be2) into dev (082c00a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #956   +/-   ##
=======================================
  Coverage   47.20%   47.20%           
=======================================
  Files           4        4           
  Lines         500      500           
=======================================
  Hits          236      236           
  Misses        264      264           

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