Use GraphQL endpoint for environments dropdown in monitoring dashboard 2/5

What does this MR do?

This MR is part 2/5 of #34021 (closed).

Overall idea

The environments dropdown currently shows only 20 items and there is no pagination or filtering. The issue is broken down into 5 MRs.

This MR

The environment dropdown in the monitoring dashboard currently uses GitLab REST API. In this MR, we're switching the API with recently introduced GraphQL API to fetch environments. The GraphQL API will help with searching and filtering environments in the dropdown.

Others MRs

Part	Description
Part 1/5 (!23017 (merged))	Added a searchable input to the environments dropdown to do client-side filtering.
Part 2/5 (!23632 (merged))	Use GraphQL endpoint to fetch environments
Part 3/5 (!23659 (merged))	Cleanup of environments REST API occurences
Part 4/5 (!23834 (merged))	Introduce a loading state and server-side filtering
Part 5/5 (Todo)	Remove the feature flag.

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

• 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