Fix metrics graphs environments dropdown

What does this MR do?

• Adds an optional parameter to the Environments#index endpoint which allows for responses nested by folders, with the default as a flat response.
• Updates environments list (Operations -> Environments) to use the new parameter.
• Updates metrics dropdown (Operations -> Metrics) to expect a flat response.

What are the relevant issue numbers?

Fixes #49388 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Tests added for this feature/bug
• [n/a] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [n/a] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer