Enable search of environments

What does this MR do and why?

Removes the FF introduced in the !97015 (merged)

I added the feature flag mostly to avoid breaking the environments page. The actual search UI will be added without the feature flag.

Screenshots or screen recordings

No visible changes

How to set up and validate locally

1. Go to the environments page, create a few environments(use folders like review/my-review...)
2. open developer console and find requests like:

• https://gitlab.com/gitlab-org/gitlab/-/environments.json?nested=true&page=1&scope=available
• https://gitlab.com/gitlab-org/gitlab/-/environments/folders/review.json?scope=available&per_page=3 (Note that I added .json here, it isn't present in the logs because JS just set the accept header instead)

1. add &search=something to them

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.

Related to #372541 (closed)