Migrate backstage labels

Create one-off policies to migrate backstage [DEPRECATED] to a go forward label based on the mapping below

1. Danger bot / static code analysis => ~"tooling" + ~"tooling::workflow"
2. ci-build => ~"tooling" + ~"tooling::pipelines"
3. regression => ~bug
4. Architecture decision / ~"technical debt" / ~"dependency update" / database / test => ~"feature" + ~"feature::maintenance"

Create one-off policy to remove backstage [DEPRECATED] label

Migration plan

1. Run each policy once in the order listed above. In terms of policy files, the order would be as follows:
   1. policies/one-off-backstage-migration/tooling-workflow.yml (dry run: https://gitlab.com/gitlab-org/quality/triage-ops/-/jobs/662969994)
   2. policies/one-off-backstage-migration/tooling-pipelines.yml (dry run: https://gitlab.com/gitlab-org/quality/triage-ops/-/jobs/662969995)
   3. policies/one-off-backstage-migration/bug.yml (dry run: https://gitlab.com/gitlab-org/quality/triage-ops/-/jobs/662969996)
   4. policies/one-off-backstage-migration/feature-maintenance.yml (dry run: https://gitlab.com/gitlab-org/quality/triage-ops/-/jobs/662969997)
2. After the above policies are ran, issues and MRs with backstage [DEPRECATED] and one of the label listed above would have been migrated.

Dry run results

Number of matching issues & MRs:

Rule	# of Issues	# of Merge Requests
Migrate Danger bot to ~"tooling::workflow"	29	102
Migrate static code analysis to ~"tooling::workflow"	62	167
Migrate ci-build to ~"tooling::pipelines"	124	596
Migrate regression to ~"bug"	15	131
Migrate Architecture decision to ~"feature::maintenance"	19	9
Migrate ~"technical debt" to ~"feature::maintenance"	1464	2059
Migrate ~"dependency update" to ~"feature::maintenance"	76	1099
Migrate database to ~"feature::maintenance"	162	1036
Migrate test to ~"feature::maintenance"	587	1726

These numbers are an over estimate. When the actual policy is run, each policy may reduce the number of issues & MRs for the subsequent policies due to one of ~tooling, ~feature, ~bug being applied.

Estimated: 55% of all issues and 64% of all MRs with backstage [DEPRECATED] would be migrated.

Reference:

• total issues with backstage [DEPRECATED] (open & closed): 4,634
• total MRs with backstage [DEPRECATED] (open & closed): 10,873

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Week-in-review

Closes #483