refactor(ui5-illustrated-message): remove titleLevel property

[yanaminkova]

Removes the titleLevel property of the ui5-illustrated-message, since users can set the required title level on the title slot.
Also, now the the slotted element has proper styles and responsiveness.

BREAKING CHANGE: The titleLevel property of the ui5-illustrated-messageis removed.
If you have previously used the titleLevel property:

<ui5-illustrated-message title-level="H6>

it will no longer work for the component.

Instead, you could set the title of the ui5-illustrated-message on the title slot, as it follows

<ui5-illustrated-message>
      <ui5-title slot="title" level="H3">This is a slotted title</ui5-title>
</ui5-illustrated-message>

Related to #8461, #7887