Owners alone can use group protected environments API

What does this MR do and why?

Change the permission model of the Group protected environments to require Owner role as discussed in #366617 (closed).

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 next 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 %15.6) even after removal of first FF so that customers can default enable it to get the old behaviour if required.

Documentation change suggested in the issue proposal will be in another MR.

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How to set up and validate locally

Follow these steps to reproduce #366617 (closed)

MR acceptance checklist

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Related to #366617 (closed)