Types: add return type declaration to jsonSerialize methods - #552

Tested this against current develop, since #634 reports the same notices on PHP 8.4 and 8.5.

• The branch is 11 commits behind develop but merges clean.
• On PHP 8.5.11 and 8.4.26, loading the 5 classes from develop gives 5 "Return type of ...::jsonSerialize()" deprecation notices. With this PR merged into develop there are none.
• PHPUnit on PHP 8.4 (wp-env): 62 tests, 84 assertions, 24 skipped, no failures. Same as develop.
• PHPCS with the current ruleset is clean. On the branch as it is, PHPCS fails because the branch still has the old phpcs.xml.dist (WordPress.CodeAnalysis.EmptyStatement is gone in WPCS 3) plus a few style issues already fixed on develop. A rebase on develop should fix that.
• PHP 7.4.33: php -l passes on all 5 files and the classes load fine. : array is valid since PHP 7.0.

The red checks here look like a CI infra problem from December, not this change. The jobs exited with code 127 before any step ran. The logs have expired, so I can't confirm more than that. A rebase should re-run CI.

One small note: these methods are not final, so a class outside the plugin that extends them and overrides jsonSerialize() without a return type would now get a fatal error. I found none in this repo, and this is a feature plugin, so it should be fine.

Could you rebase and add "Fixes #634" to the description so the issue closes on merge?