[Header locales] Support gradients (#2386)

[damiankrzyz]

• support gradien in header for locales selector

• Remove gradient from local dropdowns in header

PR Summary:

Why are these changes introduced?

Fixes #0.

What approach did you take?

Other considerations

Decision log

#	Decision	Alternatives	Rationale	Downsides
1

Visual impact on existing themes

Testing steps/scenarios

• Step 1

Demo links

• Store
• Editor

Checklist

• Added PR summary for release notes
• Requested review from UX (Only for changes that are affecting the experience or perceivable visual details)
• Created a ticket for the help.shopify.com documentation team about updates to theme settings. (Internal-only task)
• Followed theme code principles
• Linted with Theme Check
• Tested on mobile
• Tested on multiple browsers
• Tested for accessibility