Remove support of ~backstage in Danger

Proposal

There are Danger checks which refer to ~backstage that need to be updated with https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/488#proposal

gitlab-org/gitlab !36420 (merged)

• Update https://gitlab.com/gitlab-org/gitlab/-/blob/master/danger/specs/Dangerfile
• Update https://gitlab.com/gitlab-org/gitlab/-/blob/master/danger/metadata/Dangerfile
• Update https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/danger/changelog.rb

gitlab-org/gitaly gitaly!2371 (merged)

• Update https://gitlab.com/gitlab-org/gitaly/-/blob/master/danger/changelog/Dangerfile
• [ ] Update https://gitlab.com/gitlab-org/gitaly/-/blob/master/danger/labels/Dangerfile There is no more ~backstage in this file gitaly@a3c43b4f (comment 376420486)

gitlab-org/charts/gitlab gitlab-org/charts/gitlab!1442 (merged)

• Update https://gitlab.com/gitlab-org/charts/gitlab/-/blob/master/scripts/support/changelog/Dangerfile
• Update https://gitlab.com/gitlab-org/charts/gitlab/-/blob/master/scripts/support/metadata/Dangerfile

gitlab-services/version-gitlab-com https://gitlab.com/gitlab-services/version-gitlab-com/-/merge_requests/511

• Update https://gitlab.com/gitlab-services/version-gitlab-com/-/blob/master/danger/specs/Dangerfile
• Update https://gitlab.com/gitlab-services/version-gitlab-com/-/blob/master/danger/metadata/Dangerfile