Require at least Drupal 10.3 and remove any usage of deprecated classes, methods, constants, or functions

I will postpone this, for now, as I would like to first merge some MRs.

The full list of deprecation errors is the following one.

• Fetching deprecated class constant EXISTS_RENAME of interface Drupal\Core\File\FileSystemInterface: in drupal:10.3.0 and is removed from drupal:12.0.0. Use \Drupal\Core\File\FileExists::Rename instead.
• Fetching deprecated class constant EXISTS_REPLACE of interface Drupal\Core\File\FileSystemInterface: in drupal:10.3.0 and is removed from drupal:12.0.0. Use \Drupal\Core\File\\FileExists::Replace instead.
• Class Drupal\render_example\Element\Marquee extends deprecated class Drupal\Core\Render\Element\RenderElement: in drupal:10.3.0 and is removed from drupal:12.0.0. Use \Drupal\Core\Render\Element\RenderElementBase instead.
• Call to deprecated method renderPlain() of class Drupal\Core\Render\Renderer: in drupal:10.3.0 and is removed from drupal:12.0.0. Use \Drupal\Core\Render\RendererInterface::renderInIsolation() instead.

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