Doc owners alone can configure group protected environments

What does this MR do and why?

Documentation change for !92801 (merged) where we have modified the permissions for owners of group protected environments from Maintainer+ to Owner+.

We introduce this change with two FF's:

• group_level_protected_environment_settings_permission will be used for gradual rollout on gitlab.com in phases using actors this week. Expecting to have it default enabled in this milestone.
• override_group_level_protected_environment_settings_permission it should be off. Will be kept for two milestones (till %15.5) even after removal of first FF so that customers can default enable it to get the old behaviour if required.

Related to #366617 (closed)

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