Audit the current group definition for group triage reports

Now who should be assigned and mentioned were defined at lib/group_definition.rb, and some groups were outdated from the information written on Product sections, stages, groups, and categories.

One example mentioned at !483 (comment 334640221) that ~"group::threat management" was renamed to groupthreat insights, and we need to reflect that.

I'll go over it and also split engineering managers so we can reuse better for !483 (merged)

For example, change this:

def group_analytics
  {
    mentions: %w[@dennis @djensen @npost @jshackelford @wortschi],
    assignees: %w[@dennis @djensen @jshackelford @wortschi],
    labels: ['group::analytics']
  }
end

To this:

def group_analytics
  {
    pm: %w[@jshackelford],
    em: %w[@wortschi @djensen],
    product_design: %w[@npost],
    labels: ['group::analytics']
  }
end

I am confused if @dennis is a frontend engineering manager for this group, so I think I might need to ask a lot of people for confirming the new list...

/cc @gl-quality/eng-prod