Migrate `group_icon` to `Pajamas::AvatarComponent` to support `srcset`

Background

A lot of avatars rendered with HAML use the group_icon helper to render the avatar. This helper does not support adding the srcset attribute to improve rendering of images on retina screens. We have Pajamas::AvatarComponent that does support adding the srcset attribute out of the box so we should migrate to that.

Files to migrate

• app/views/groups/_home_panel.html.haml#L9
• app/views/shared/groups/_group.html.haml#L7
• ee/app/views/groups/sso/_register_pane.html.haml#L9
• ee/app/views/shared/billings/_billing_plans_layout.html.haml#L4
• ee/app/views/shared/billings/_billing_plans_layout.html.haml#L7
• app/helpers/groups_helper.rb#L214

Implementation guide

See Migrate HAML project avatars to `Pajamas::Avata... (!135985 - merged) for a starting point