feat: refactor pr.canRebase => !pr.isModified (#4385)

lib/platform/azure/azure-helper.ts

lib/platform/bitbucket-server/index.ts

lib/platform/bitbucket-server/utils.ts

lib/platform/bitbucket/index.ts

lib/platform/github/index.ts

lib/platform/gitlab/index.ts

lib/workers/branch/index.js

lib/workers/branch/parent.js

lib/workers/pr/index.js

lib/workers/repository/finalise/prune.js

lib/workers/repository/onboarding/branch/rebase.js

lib/workers/repository/onboarding/pr/index.js

test/platform/azure/__snapshots__/azure-helper.spec.ts.snap

...latform/bitbucket-server/__snapshots__/index.spec.ts.snap

test/platform/bitbucket/__snapshots__/index.spec.ts.snap

test/platform/github/__snapshots__/index.spec.ts.snap

test/platform/github/index.spec.ts

test/platform/gitlab/__snapshots__/index.spec.ts.snap

test/workers/branch/index.spec.js

test/workers/branch/parent.spec.js