Replace cycle analytics group labels endpoint

What does this MR do?

Follow on from: !24218 (comment 287365367)

We seem to have two endpoints for group labels:

• UI: http://localhost:3001/groups/gitlab-org/-/labels (add .json)
  • Includes project labels
• API: http://localhost:3001/api/v4/groups/gitlab-org/labels.json
  • Only group labels

This MR update cycle_analytics to fetch the labels from http://localhost:3001/api/v4/groups/gitlab-org/labels.json, as we do not want to include project-level labels. This ensures that a selected label is available to be used as a label event in the custom stage form.

• Adds a fixture spec to generate the correct labels for /api/:version/group/:groupid/labels data
• Adds snapshot tests so we can catch missing label titles earlier
• Consolidates analytics/value_stream_analytics urls in mock_data

This is all behind the :customizable_cycle_analytics feature flag

Screenshots

Before - includes project labels	After

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Style guides

Availability 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
• 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