chore: use ::class syntax to fetch class name instead of get_class() function

[datlechin]

This pull request updates the codebase to use the ::class syntax for obtaining the fully qualified class name from objects. This improvement aligns with the introduction of this syntax in PHP 8.0.

See more: https://wiki.php.net/rfc/class_name_literal_on_object

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)
• Related core extension PRs: (Remove if irrelevant)

[SychO9]

Thanks!

[luceos]

What a simple, but useful addition. @datlechin thank you 🙏