Introduce migrate_environment_details_to_vue feature flag [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This MR creates the migrate_environment_details_to_vue feature flag and displays an empty page when it's enabled. The goal is migrate the page to vue MR by MR and this is the first MR which introduces the feature flags and moves the page content behind it. By default, the feature flag is disabled and the current implementation is shown.

Screenshots (strongly suggested)

No visual changes.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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

Related to #254258 (closed)