Rename "Deployments > Feature Flags" to "Deployments > Feature flags"

Rename "Deployments > Feature Flags" to "Deployments > Feature flags"

Part of menu reorganization in #393190+s

We need to rename "Deployments > Feature Flags" to "Deployments > Feature flags", in order to respect the new menu structure defined in https://gitlab.com/gitlab-org/gitlab/-/issues/393190(expand the Projects section in the description), and see this comment for more details.

Closes #404074 (closed)

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