[Discuss] Undeprecate use of percentage-of-time Feature Flags

What does this MR do?

Not necessarily meant to be merged, just trying to get feedback for now!

We had deprecated use of percentage-of-time FFs: https://docs.gitlab.com/ee/development/feature_flags/controls.html#percentage-of-time-roll-out

The reasoning was that in face of deploying new features, a user should experience consistent behavior so that the same user always falls into the same on/off bucket.

I think this is a valid thing to say, but misses several other use cases where pct-of-time FFs would lead to better user experience in the presence of failures, or are even necessary:

1. gitlab_com_derisk rollouts: These kinds of feature flags do not typically introduce new features. They are put in place to de-risk a structural change with no visible effect to users such as refactors. If there were errors during the rollout, using pct-of-actors leads to dramatically worse user experience: when rolled out to say 25%, 1 in 4 users on our website will experience 100% of these failures i.e. essentially a complete outage on this code path. With pct-of-time, only 1 in 4 requests would fail for any given user.
2. Not all code paths can be allocated to an actor. There are valid use cases where the implementation has no actor associated with it. A cron job is a good example: if it performs cleanup or pre-caching, to name just two examples, what user or group would this be associated with?

I think we should un-deprecate percentage-of-time FFs and let developers decide what is the best tool to get their job done.

Related issues

MR that spawned this discussion: !172378 (merged)

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