Removes support for php 7.1

[micheleorselli]

This is a first step in order to drop support for php 7. Here I am removing support php 7.1 which was causing problems in the pipeline due to old symfony/process versions with security bugs and a too much wide version for phpcsfixer which resulted in different version installed based on the php version, leading to inconsistent results

This would also unblock the merging of #444.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.06%. Comparing base (cc81afc) to head (1eea832).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #446   +/-   ##
=========================================
  Coverage     94.06%   94.06%           
  Complexity      601      601           
=========================================
  Files            69       69           
  Lines          1551     1551           
=========================================
  Hits           1459     1459           
  Misses           92       92           

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