Adopt the PHP 8.1 syntax Rector proposes and remove its skips #601

Summary

Apply the PHP 8.1 syntax that Rector's 8.1 level set proposes (readonly properties and first-class callable syntax), and remove the three skips that keep composer rector:check green until then. Part of the PHP 8.1 update (#595).

Why

#595 moves Rector to the 8.1 level set. Three of its rules would change 52 files, so they are skipped for now:

• ReadOnlyPropertyRector: 84 properties in 48 files become readonly.
• FunctionFirstClassCallableRector: 5 places ('strlen' becomes strlen(...)).
• ArrayToFirstClassCallableRector: 2 places ([$this, 'method'] becomes $this->method(...)).

Applying all three at once passes composer cs:check and composer phpstan, but breaks 4 tests, so each rule needs a decision.

Findings from a trial run

• MigrationManagerTest (2 tests) replaces the db property by reflection. A readonly property cannot be changed after the constructor.
• ErrorHandlerTest (2 tests) checks that ErrorHandler::setup() registers [$handler, 'handleError'] and [$handler, 'handleException']. After the rewrite PHP holds a Closure instead of an array, which is a visible change in what the registered handler is.
• readonly also changes how the classes behave: a property can no longer be reassigned, which matters to anyone who subclasses them.

Proposed change

Take the rules one at a time, one commit each:

• FunctionFirstClassCallableRector: apply the 5 edits.
• ArrayToFirstClassCallableRector: apply in Transformer; decide for ErrorHandler whether to keep the array callable (and skip that line) or change the tests.
• ReadOnlyPropertyRector: apply, change the tests that set those properties by reflection, and list in the changelog the classes whose properties became readonly.
• Remove each skip from rector.php when its rule is applied.

Out of scope

• Turning AppType and LoggerType into enums.
• Anything that needs PHP 8.2 or newer.

Breaking changes

• Subclasses that reassign a property that became readonly. To be listed in the changelog.

Acceptance criteria

• rector.php has no skip for these three rules, or a comment for each one that stays
• composer test, composer cs:check, composer phpstan and composer rector:check pass on every PHP version in the CI matrix

Notes

Relevant code: rector.php, src/Tracer/ErrorHandler.php, tests/Unit/Migration/MigrationManagerTest.php, tests/Unit/Tracer/ErrorHandlerTest.php.