chore: add rector

[phil-davis]

Doing this first with the code that still supports PHP 7.4.
That will make it easier when backporting any future patches - the starting point of the 7.0 minor version code will be close to what is in master.

rector only made small changes to production code. Most of the changes were in test code.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.15%. Comparing base (4386c45) to head (03ddb4c).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #278      +/-   ##
============================================
- Coverage     94.21%   94.15%   -0.06%     
  Complexity      263      263              
============================================
  Files            15       15              
  Lines           881      873       -8     
============================================
- Hits            830      822       -8     
  Misses           51       51              

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