Update Environment setting page to use GraphQl for fetching data

What does this MR do and why?

As part of allowing to select an agent in the Environment page, this MR updates the edit_environment component to use GraphQl to get environment data. We'll update it to use GraphQl mutation for storing the changes in a separate MR.

Screenshots or screen recordings

No visual changes, the screenshot for the context:

How to set up and validate locally

1. Visit Project -> Build -> Environments page
2. Create an environment using the UI
3. Update the environment

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Review progress:

• backend developer review
• backend maintainer review
• frontend developer review
• frontend maintainer review

Related to #396407 (closed)