Add server-side search for environments dropdown

What does this MR do?

Environments dropdown in the monitoring dashboard has a search input behind searchable_environments_dropdown feature flag. The search input currently only filters client-side. This MR allows the environments dropdown to hit the GraphQL endpoint to do server-side filtering.

Note 1 Because all the changes are behind a feature flag there is no changelog file.

Note 2 The CSS change in this MR was originally added here !23017 (comment 273789733) and was approved by the UX. However, because of another MR (listed in the comments) the change got removed it.

Screenshot

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

Mentions #34021 (closed)