Disable and consider rolling back Adoption report

Edit: the two concerns raised in this issue have been transferred to two separate epics to track progress. See &5251 (closed) and &5252 (closed).

Problem

In 13.7, we shipped the Adoption page with Segments (https://docs.gitlab.com/ee/user/admin_area/analytics/dev_ops_report.html#devops-adoption). This feature seems to have a few problems:

1. Introduces complexity through configuration. Like @ebrinkman and I described in this thread, we're introducing a new concept before we allow users to simply do their reporting on groups.

2. Instance-level feature in the admin panel. We need to start building things at the group level and dig ourselves out of the admin panel.

Proposal

• Consider the adoption report a beta feature
• In 13.8, rename segment to group, backport to 13.7 if possible, and limit the number of groups that can be selected to one group.

• In 13.8, Feature flag the Adoption report and disable it by default

• In 13.8, Enable adoption report by default if segments already exist
• In 13.9, migrate existing segments to the group structure
• In 13.9, eliminate the ability to apply a name (formerly segment name) when selecting a group to add to the table (exact approach still being discussed)
• Create a proposal for how to add this feature to the group level