The warning_text could be more visible

Problem/Motivation

As I was editing a block in the layout builder, I wondered where the warning text was visible. Then I saw it at the top of the modal.
As a contributor, I guess (s)he won't know the text exists as (s)he didn't configure the module by him(her)-self.
I propose to make this message visible as a warning text, to prevent the user from modifying or deleting the block.

Steps to reproduce

1. Migrate Drupal from 10.6 to 11.3.9
2. Add the layout_builder_reusable_blocks module with composer
3. enable the layout_builder_reusable_blocks module
4. enable the dblog module
5. in the configuration menu, configure the layout_builder_reusable_blocks module as following

- check "Show warning message when editing reusable blocks"
- check "Allow editing reusable blocks"
- check "Make all blocks reusable by default"

6. Save the configuration
7. Go to an existing node, in layout mode
8. Choose an existing block, contextual menu > configure

Result: the message is not layed out.

Proposed resolution

Update the form message as a warning.