Raise min version to PHP 7.3, check compatibility PHP 8.3.

[terabytesoftw]

Q	A
Is bugfix?	❌
New feature?	✔️
Breaks BC?	❌

[what-the-diff]

PR Summary

• Indentation Standards Enforced in YML Files
  The size of indentations in all *.yml files in this project has been standardized to two spaces.

• Narrowed Down GitHub Workflow Build Triggers
  The build process on GitHub won't trigger unnecessarily, as changes in less-important files and directories are now ignored.

• Upgraded Testing Suite to Latest Version
  The 'PHPUnit' testing tool used in this project has been upgraded to its latest version, thus providing more reliable and up-to-date testing mechanisms.

• Streamlined Resource Loading for Tests
  The tests now use a centralized setup file, ./tests/bootstrap.php, which simplifies the organization and maintenance of test scripts.

• Mandated Return Type for Test-Setup Methods
  Several test files' setup methods have been upgraded to declare a return type of "void". These changes further clarify the purpose of these methods, helping ensure correct usage.

• Improved Test Cleanup Operations
  The teardown method in general 'TestCase.php' now has a clarified return type of "void", which shows it doesn't output anything, and is used for cleanup after each test run.

• Removed Redundant Compatibility File
  The 'compatibility.php' file and all references to it have been removed for leaner, cleaner code.

In summary, this PR improves the clarity, reliability, and overall quality of the project’s testing suite while cleaning up the codebase by removing redundant files and setting precise standards.

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