Filtered search width fix on Roadmap

What does this MR do?

Fixes the sizing of Filtered search on Epic Roadmap.

Issue: gitlab-ui#908 (closed)

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Conformity

• Changelog entry
• Documentation (if required)
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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
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Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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