chore: drop the need for a json-api-server fork

[SychO9]

Part of #3964

Changes proposed in this pull request:
With the recent changes to the json-api-server package made by Toby, it is easier to not rely on the package, though we still have to apply many custom changes in core itself, it is better than maintaining a fork and the custom changes are likely to decrease as the package becomes even more flexible in time.

Also the fork technicaly sill exists, but is 1:1 to the original package, until the original package is tagged with the latest changes Or we might always keep it manually 1:1 synced in case of any urgent changes.

Reviewers should focus on:

Screenshot

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)