refactor: upgrade to PHP 8.1 with rector

[kenjis]

Description
Supersedes #6923
See #6921 and #8042

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[kenjis]

@samsonasik When I ran rector first, the following change did not happen.
I ran rector again, I got the change.

$ vendor/bin/rector 
 119/119 [▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓] 100%
1 file with changes
===================

1) system/Router/AutoRouterImproved.php:410

    ---------- begin diff ----------
@@ @@
     {
         try {
             $refClass = new ReflectionClass($this->controller);
-        } catch (ReflectionException $e) {
+        } catch (ReflectionException) {
             throw PageNotFoundException::forControllerNotFound($this->controller, $this->method);
         }
    ----------- end diff -----------

Applied rules:
 * RemoveUnusedVariableInCatchRector (https://wiki.php.net/rfc/non-capturing_catches)

[samsonasik]

@kenjis rector sometime need twice run as the code got consecutive run on single rule or just reprinted on next rule apply to specific node to avoid infinite loop

[kenjis]

@samsonasik I got it. Thanks!

[kenjis]

Rebased and re-run rector.

[MGatner]

I'm not going to review all the changes, but what I spot-checked looks good and pipeline is passing.

This will inevitably be a big merge conflict before we are ready for the next minor release. I suggest you get the automated changes in place via Rector (done) then consolidate the manual revisions to a commit(s) that can be cherry-picked after rerunning Rector.

Excited to be on 8+! Thanks for your work on this - and shoutout to the attention from @TomasVotruba 😍 and the tireless effort from the whole Rector team, including our own @samsonasik.

[kenjis]

@MGatner I don't understand what you exactly mean.
I did atomic commits. Do you suggest to combine the commit for refactor by Rector and the commits for manual tweaks into one commit?

[MGatner]

No, sorry to be unclear! I think what you are doing is great - I was just noting that it might not be worthwhile to keep up with conflicts on this PR, but rather wait and rerun Rector right before we are ready for release.

[kenjis]

@MGatner I got your point. Thanks.

[github-actions]

👋 Hi, @kenjis!

We detected conflicts in your PR against the base branch 🙊
You may want to sync 🔄 your branch with upstream!

Ref: Syncing Your Branch

[github-actions]

👋 Hi, @kenjis!

We detected conflicts in your PR against the base branch 🙊
You may want to sync 🔄 your branch with upstream!

Ref: Syncing Your Branch

[kenjis]

Due to the difficulty in resolving the conflicts, I close this PR.
Go to #8354.