Swimlanes - Fix url encoding when adding group_by param

What does this MR do?

Fixes bug where labels with spaces get duplicated on page refresh after switching to swimlanes view on boards.

Replaces setUrlParams with mergeUrlParams to use encodeURIComponent encoding logic, which is used when populating URL with board scope filters.

Require feature flag :boards_with_swimlanes.

Screenshots (strongly suggested)

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

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• 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