Simplify styling for 'Search bar'

[janfaracik]

This PR updates and simplifies the styling for the search bar component, where rather than reinvent the wheel in terms of design we just inherit the design from the existing input and dropdown classes.

This results in a mild UI change for the search bar, but keeps it consistent with other inputs:

Before

After

I've also simplified results for the search bar, we now reuse the styling of the dropdown component rather than have our own unique style for search.

Testing done

• Tested command palette, regular search and plugin search
• Results appear as expected

Proposed changelog entries

• N/A

Proposed upgrade guidelines

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2. The changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developers, depending on the change) and are in the imperative mood (see examples). Fill in the Proposed upgrade guidelines section only if there are breaking changes or changes that may require extra steps from users during upgrade.
3. There is automated testing or an explanation as to why this change has no tests.
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7. For dependency updates, there are links to external changelogs and, if possible, full differentials.
8. For new APIs and extension points, there is a link to at least one consumer.

Desired reviewers

@jenkinsci/sig-ux

Before the changes are marked as ready-for-merge:

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3. Changelog entries in the pull request title and/or Proposed changelog entries are accurate, human-readable, and in the imperative mood.
4. Proper changelog labels are set so that the changelog can be generated automatically.
5. If the change needs additional upgrade steps from users, the upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the pull request title (see example).
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