Raise PHP version requirement

[JeroenDeDauw]

This brings it in line with SMW and with the CI

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 45.52%. Comparing base (f13705e) to head (9788b7e).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #927   +/-   ##
=========================================
  Coverage     45.52%   45.52%           
  Complexity     2319     2319           
=========================================
  Files            80       80           
  Lines          9050     9050           
=========================================
  Hits           4120     4120           
  Misses         4930     4930           

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