Fix License Compliance widget button spacing

What does this MR do?

Fix License Compliance widget button spacing

Use flexbox for laying out the buttons, to avoid whitespace in the template from affecting visual spacing.

This also replaces the deprecated slot syntax with modern shorthand.

Addresses #283889 (closed).

Screenshots (strongly suggested)

Before
After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Related to #283889 (closed)