Revert #263, fixes jsonSerialize fatal error on php 7.4 or lower

[menno-ll]

Issue link

https://level-level.slack.com/archives/C02RENN0T/p1700146322403449
https://app.asana.com/0/1194139319537345/1205063740343242/f

Description

Reverts PR #263 .
This is needed because now already changing the return type would cause a fatal in PHP 7.4 or lower.
Instead, PR #265 has been created to bypass the deprication notice in PHP 8.1+, till Clarkson Core will not support PHP versions below 8.

How Has This Been Tested?

I picked a project running php 7.4.
And updated clarkson core in this project.
Then i added the following line to functions.php: echo json_encode( post::get(1) );. This breaks with the error shown in screenshot.
Then I removed the mixed return type from the jsonSerialize function.
The error is now gone.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.