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

• Workflow specificity improved
  This PR improves how our system responds to changes in our code. Certain less relevant files have been set to be ignored, meaning they won't trigger a full system operation when they are modified. This makes our development process more efficient by not wasting resources on insignificant changes.

• Dependency and PHP version update
  To ensure our application is up to date and secure, we've increased the minimum required PHP version to 7.3. Along with this, the 'phpunit/phpunit' dependency (one of the software's necessary components) has been updated to version ^9.6.

• Refinements in Test Cases
  Various test files have been updated to refine how they operate. One significant change is that the 'setUp' method across files now includes a return type declaration- a change that increases the clarity of what the method should return. Another significant change is the replacement of 'assertContains' assertions with 'assertStringContainsString'. This change enhances the precision of our testing system.

• File Deletion and Changes
  Unused files like 'tests/compatibility.php' have been removed, and any references to these files from others have also been adapted accordingly. This removal streamlines our codebase and avoids any potential confusions from outdated files.

[codecov]

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