Fix Drupal 11 deprecations and remove Drupal < 10.3 support

Let's create a 4.x branch that drops Drupal 9 support and requires ^10.3 at least.
We'll also use that 4.x branch for new features then.

Further fixes required:

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  Line   field_group.module                                                    
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  358    Call to static method isDisabled() on an unknown class Drupal\ds\Ds.  
         💡 Learn more at https://phpstan.org/user-guide/discovering-symbols   
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  Line   tests/src/Functional/ManageDisplayTest.php                                
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  94     Access to an undefined property                                           
         Drupal\Tests\field_group\Functional\ManageDisplayTest::$group.            
         💡 Learn more:                                                            
            https://phpstan.org/blog/solving-phpstan-access-to-undefined-property  
  98     Access to an undefined property                                           
         Drupal\Tests\field_group\Functional\ManageDisplayTest::$group.            
         💡 Learn more:                                                            
            https://phpstan.org/blog/solving-phpstan-access-to-undefined-property  
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All green, finally! Please review!

GREAT! :)

After merge tests now fail again.

Ok, the tests I did for ✨ Add option to allow HTML in labels Fixed were not properly finished, as they didn't render the tabs probably and it seems I never commented out the tabs that do not render:

// See if the field group supports HTML elements in the label:
// Note, for some reason only Tab 2 gets rendered on the page:
// We expect the HTML to be not escaped:

So now in 1 Test it renders in the horizontal tabs view, but in the other it doesn't, but both check the same css class structure, which is different for one test.

I'll fix this temporarily and spend a couple of minutes to see if I can fix the tests for good.

Alright, the new phpcs errors are due to commented out test code and the phpunit failures are due to 🐛 Horizontal Tabs render broken, if child allows HTML Active . We need to fix them in the mentioned issue.

But everything else here is ready to being merged.