Change color pickers set-color prop to value so it links better with Vue and input standards

What does this MR do?

This MR changes the set-color prop to value. This is more standard for input types in Vue and also means that you can use things like .sync and other Vue "magic".

Screenshots (strongly suggested)

No changes

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

This component isn't used anywhere visible just yet (the shared form isn't added to any pages right now 🙈) but you can locally add it to an existing Vue instance:

<color-picker
     :invalid-feedback="__('Please enter a valid hex (#RRGGBB or #RGB) color value')"
     :label="__('Background color')"
     :value="#FF0000"
     state="isValidColor"
   />

• 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

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] 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