Modify feature flag instructions

What does this MR do and why?

Based on the discussion here (Slack, internal-only):

• ops environment is not equipped to run chatops commands yet, so we need not have it mentioned in the feature flag rollout template. It is only going to be introduced at a later time: gitlab-com/gl-infra/delivery#2875. Running chatops commands in ops currently leads to a failure, so it is best to get this removed to prevent confusion.

• pre environment can be used to test feature flag changes, but our rollout template currently does not include the instructions to enable the flag on pre. However, it has the instructions to delete the flag from pre. To make things consistent, this MR changes the template such that it includes the instructions to both enable and disable flags in pre

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