Improve code base with Rector

[Art4]

This PR adds rector as a new dev dependency to update the code base automatically.

Yesterday PHP 8.5 was released, so I let the tests run on PHP 8.6 (dev) as well.

[codecov]

Codecov Report

❌ Patch coverage is 97.50000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.61%. Comparing base (5a38b69) to head (31dc830).

Files with missing lines	Patch %	Lines
src/Redmine/Client/NativeCurlClient.php	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               v2.x     #455      +/-   ##
============================================
- Coverage     98.63%   98.61%   -0.03%     
+ Complexity      659      658       -1     
============================================
  Files            29       29              
  Lines          2056     2022      -34     
============================================
- Hits           2028     1994      -34     
  Misses           28       28              

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[Art4]

while working on this I noticed that some rector rules would introduce breaking changes, because rector is not aware of a "library concept", where non-final classes could be overridden by a downstream project.

My first approach was to find those specific rules and skip them. But then I had the same problem with another library. So I decided to bundle theses efforts and created a new set of backward compatible rules specific for library maintainers. (aka new side project 😊 )

https://packagist.org/packages/art4/rector-bc-library

[kbsali]

👏 awesome!!!