Raise minimum PHP version to 8.1 and refactor code

[vjik]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	❌
Fixed issues	-

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.65%. Comparing base (749cefa) to head (e4c79cd).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #85 +/- ## ============================================ - Coverage 99.65% 99.65% -0.01% + Complexity 107 102 -5 ============================================ Files 11 11 Lines 290 288 -2 ============================================ - Hits 289 287 -2 Misses 1 1 ```

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[what-the-diff[bot]]

PR Summary

• Updates on Workflow Configurations The pull request modifies the workflow configurations present in multiple files in the .github/workflows directory. These configurations now consider specific paths while initiating actions during pull requests and pushes. Also, certain branches have been specified for push events. A new secret has been added for mutation.yml.

• Addition of New Workflow Configurations A new workflow configuration file, rector.yml, has been added in the .github/workflows directory.

• Updates on Project Documentation and Requirements The CHANGELOG.md and README.md files have been updated for better documentation. The PHP version requirement has been updated to 8.1 or higher and the requirement for the JSON PHP extension has been removed. This alteration is also reflected in composer.json file and new dependencies were added.

• Ignored Rules for specific issues The psalm.xml file was updated to ignore specific issues.

• Addition of a new rectification file A new rectification file, rector.php, has been added to the project files.

• Updates on DataResponse Properties and Methods The DataResponse.php file and its methods have been thoroughly modified to use a private $data property.

• Modification of Various Files and Methods Several files, namely DataResponseFactory.php, HtmlDataResponseFormatter.php, XmlDataResponseFormatter.php, ContentNegotiator.php, FormatDataResponse.php, have been revised. Modifications range from type hint adjustments to replacement of string position checking methods.

• Updates on Test Files and Methods The test files DataResponseTest.php, XmlDataResponseFormatterTest.php, ContentNegotiatorTest.php, TestCase.php, have been altered to adapt to the changes made in the corresponding primary files.