Remove redundant ext-json requirement

[asllanmaciel]

Removes the redundant ext-json Composer requirement.

PHP JSON has been part of core since PHP 8.0 and cannot be disabled, while this package already requires PHP >= 8.2. Keeping ext-json in require therefore adds no compatibility protection.

Verification:

• composer validate --strict --no-check-publish — pass
• composer test:typing — pass (0 errors)
• composer test:syntax — pass
• composer test:syntax_tests — pass
• git diff --check — pass
• composer test:unit_offline still reports the same pre-existing data-provider error reproduced on upstream master before this change: 44 tests, 110 assertions, 1 error, 6 skipped (Subscription::__construct() receives false for the endpoint). This patch does not change that result.

AI assistance was used to help prepare and verify this contribution; the final one-line change and verification results were reviewed directly.

Fixes #461.

[Minishlink]

Thanks!