Resolve "Default bar colours for Insights"

What does this MR do?

This change the default behavior for the Insights bar chart for time series to use a single color #428bca and use the apricot color #ffd8b1 for the last bar in the time serie.

This also fixes the use-case of a bar chart that is not for a time serie, as this wasn't working before.

Name	Example
bar
bar (time series, i.e. when group_by is used)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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

Closes #11186 (closed)