Restore CI for PHP 7.1+ with versioned PHPUnit configs and PHPStan cleanup

[Copilot]

• Fix phpunit.xml.dist (PHPUnit 9+) - exclude JsonSerializableReturnTypeTraitPhp81.php from coverage scan
• Fix phpunit.legacy.xml.dist (PHPUnit 8) - same exclusion added to whitelist/exclude

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This change is 

[coveralls]

Coverage is 97.891% — copilot/fix-github-ci-errors into master. No base build found for master.

[voku]

@copilot will the Stringy::jsonSerialize() change trigger a deprecated notice or something like that on newer php versions? If yes, how to handle that?

[Copilot]

@copilot will the Stringy::jsonSerialize() change trigger a deprecated notice or something like that on newer php versions? If yes, how to handle that?

Yes — without extra handling PHP 8.1+ emits a deprecation for JsonSerializable::jsonSerialize() when the return type is omitted. I fixed that in 0611d9e: PHP 7.1 still uses the plain implementation, while PHP 8.1+ loads a small compatibility trait that adds #[\ReturnTypeWillChange], so newer runtimes stay quiet without breaking old ones. I also rechecked it with E_ALL, the focused testJsonEncode PHPUnit run, and phpstan.