fix: drop appSlug configurability (#4930)

lib/config/app-strings.ts

lib/config/definitions.ts

lib/platform/azure/index.ts

lib/platform/bitbucket/index.ts

lib/platform/github/index.ts

lib/workers/branch/index.ts

lib/workers/branch/parent.ts

lib/workers/branch/status-checks.ts

lib/workers/pr/body/controls.js

lib/workers/repository/finalise/validate.js

lib/workers/repository/onboarding/branch/check.ts

lib/workers/repository/onboarding/branch/config.ts

test/platform/bitbucket-server/index.spec.ts

test/platform/utils/__snapshots__/pr-body.spec.ts.snap

test/platform/utils/_fixtures/pr-body.txt

test/workers/branch/index.spec.js

test/workers/branch/parent.spec.js

test/workers/pr/__snapshots__/index.spec.ts.snap