[issue-415809] Replace tertiary button in navigation to primary button

What does this MR do and why?

Replaced the tertiary search button in navigation with primary button in navigation section. The replaced button will be present below the Issues, MR and TODOs count buttons as per the design mentioned in this issue. There is no changes made in the functionality just the layout changes.

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Run the setup locally and then verified if the layout changes matches the design. Tried change the color preferences as well.

MR acceptance checklist

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