Swimlanes - Fix mobile view of filtering bar

What does this MR do?

Improve mobile view of filtering bar when Group By dropdown is shown.

Require feature flag :boards_with_swimlanes.

Previous iterations of Swimlanes:

Iteration	Link
Iteration 1	!32739 (merged)
Iteration 2	!33469 (merged)
Iteration 3	!33909 (merged)
Iteration 4	!34366 (merged)
Iteration 5	!35490 (merged)
Iteration 6	!36016 (merged)
Iteration 7	!38258 (merged)
Iteration 8	!39054 (merged)
Iteration 9	!39183 (merged)
Iteration 10	!40221 (merged)
Iteration 11	!40344 (merged)
Iteration 12	!41542 (merged)
Iteration 13	!41054 (merged)
Iteration 14	!41265 (merged)
Iteration 15	!42320 (merged)
Iteration 16	!42430 (merged)
Iteration 17	!43642 (merged)
Iteration 18	!43803 (merged)
Iteration 19	!44677 (merged)
Iteration 20	!45108 (merged)
Iteration 21	Current MR

Screenshots

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #238931 (closed)