[v3] chore: add rector and run in CI

[phil-davis]

For this PR, rector is set to use PHP 7.4 code,
so that this code can be released with PHP 7.4.
Then we can drop PHP 7.4 support.

Backport of #317 to v3

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.87%. Comparing base (e0ea808) to head (46b0087).
⚠️ Report is 3 commits behind head on v3.

Additional details and impacted files

@@             Coverage Diff              @@
##                 v3     #320      +/-   ##
============================================
- Coverage     96.88%   96.87%   -0.02%     
  Complexity      117      117              
============================================
  Files            13       13              
  Lines           482      480       -2     
============================================
- Hits            467      465       -2     
  Misses           15       15              

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