Drupal 11 compatibility fixes for custom_elements 3.x

MR was opened against 2.x branch instead of 3.x branch. I fixed that.

There is also a conflict with the symfony/serializer:

Fatal error: Declaration of Drupal\custom_elements\CustomElementNormalizer::supportsNormalization($data, $format = null): bool must be compatible with Symfony\Component\Serializer\Normalizer\NormalizerInterface::supportsNormalization(mixed $data, ?string $format = null, array $context = []): bool in /var/www/html/web/modules/contrib/custom_elements/src/CustomElementNormalizer.php on line 27

I tested with 10.2 (including the changed function definitions in the normalizer, on top of the still-old interface, just in case).

It works if we revert the renderInIsolation() change, and I would prefer that for the moment. (We can live with deprecation warnings until 3.1.)

Also: please check phpcs because these are all new. (I do not recognize some of them; maybe they are just new because of D11 inclusion / an updated coder module).

- put renderPlain back in place
- fix phpcs errors

Hm, strange: gitlab is suddenly expecting the "use" statements to be sorted case sensitively instead of insensitively. (So the fix commit fixed things for the current CI tests, and 'broke' my current version of phpcs/coder.) I find that change surprising, but will try to understand it later. If gitlab's happy, I'm happy.

I also dropped the video paragraph/media type, per the issue description, and updated the custom_elements_thunder README.md about why. (Code we don't want to support, depending on unsupported module.)

The "composer (next major version)" build now still does not work, because media_entity_twitter (a dev dependency) has no D11 release yet. But that can have a D11 release easily / can be figured out later without having to drop code.

So: merged, thank you!

Automatically closed - issue fixed for 2 weeks with no activity.