chore: remove redundant ext-json requirement (bundled since PHP 8.0)

[subhaushsingh]

• Resolves: Useless ext-json requirement #58354

Summary

Since PHP ^8.2 is required, the JSON extension has been bundled by default (since PHP 8.0).
The explicit "ext-json" requirement in composer.json is therefore redundant and has been removed.

This change does not introduce any functional modification.

TODO

• ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included (not required for this change)
• Screenshots before/after for front-end changes (not applicable)
• Documentation (manuals or wiki) has been updated or is not required (not required)
• Backports requested where applicable (not applicable)
• Labels added where applicable (maintainers will add)
• Milestone added for target branch/version (maintainers will add)

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[joshtrichards]

Should be fine. Basically counterpart to to #55063