Remove rector

[mfn]

Summary

I mentioned this already in the past in issues and wherenot that I suggest we remove rector as a dependency:

• it was used as a "one time thing" to upgrade the code to PHP 7.4
• it's not included in any CI workflow
• it's an old version and conflicts with other packages
  Random example: with Laravel 11 it never installed L11 locally because of dependencies of rector conflicting with other L11 dependencies, always downgrading it to L10

Of course an alternate approach could be change all of the points I raised above, but since it's introduction no one cared and with L11 we actually had to work around it, so I propose we remove it for now. If someone later wants to make a more maintained integration, so be it.