Cleanup PhpStan test

PHPStan is now green as well. I've added a couple of @todo in the code where the clean-up identified some obviously broken functionality which I wasn't able to resolve right away.

This has been committed to the module already, should this just be closed along with https://www.drupal.org/project/fullcalendar/issues/3472260 📌 Cleanup PHPCS test Needs review ?

I'm setting this issue back to active because there was a commit for it, but there's more work to do. The PhpStan (next minor) test is failing. It's allowed to fail, which is fine. But someone needs to resolve any problems with it.

Hi there, I have solved all the issues with phpstan and phpcs in this project expect one.

```
------ -----------------------------------------------------------------------
Line src/Plugin/fullcalendar/type/FullCalendar.php
------ -----------------------------------------------------------------------
1766 Relying on entity queries to check access by default is deprecated in
drupal:9.2.0 and an error will be thrown from drupal:10.0.0. Call
\Drupal\Core\Entity\Query\QueryInterface::accessCheck() with TRUE or
FALSE to specify whether access should be checked.
💡 See https://www.drupal.org/node/3201242 →
------ -----------------------------------------------------------------------
```
we get the above error when `$tids = $query->execute();`

```
------ -----------------------------------------------------------------------
Line src/Plugin/fullcalendar/type/FullCalendar.php
------ -----------------------------------------------------------------------
1766 Call to method Drupal\Core\Entity\Query\QueryInterface::accessCheck()
with true will always evaluate to true.
🪪 method.alreadyNarrowedType
💡 Because the type is coming from a PHPDoc, you can turn off this
check by setting treatPhpDocTypesAsCertain: false in your
phpstan.neon.
------ -----------------------------------------------------------------------
```
and the above when `$tids = $query->accessCheck(TRUE)->execute();`

Sounds like this is ready for review then?

Thanks for the additional work here. Great to have both phpstan tests passing.

Thanks for solving the phpstan (minor) issue.