Remove searchable environments dropdown feature flag

What does this MR do?

The environments dropdown shows only 20 items at a time. A search input is added to the dropdown to search and filter in cases where there are many environments (> 20). This is usually the case in www-gitlab-com.

This MR removes searchable_environments_dropdown feature flag.

Note: I also had to update the dropdown's header font-size in this MR. I know it is not ideal but the change is very small and I've kept the commits separate in case of a revert.

Screenshots

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)