Remove network_policy_management feature flag

What does this MR do?

This commit removes feature flag from the new Policy tab of the Threat Management page. Corresponding docs were also added.

This feature flag was introduced in !31289 (merged). All necessary features for the Policy Management MVC were implemented so we want to remove this feature flag in preparation for the release.

Issue for the Policy Management UI: &3328 (closed)

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
• 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