Run `db:` CI jobs in decomposed mode

In !68232 (merged), we added support to run RSpec jobs for decomposed databases. We should the same, but for db: CI jobs.

Note that because we need to support single, and decomposed jobs for some time, we probably need to run both variants of the db: CI jobs as well.

The list of db: jobs:

$ gg ^db: .gitlab/ci
.gitlab/ci/rails.gitlab-ci.yml:db:rollback:
.gitlab/ci/rails.gitlab-ci.yml:db:migrate:reset:
.gitlab/ci/rails.gitlab-ci.yml:db:migrate-from-previous-major-version:
.gitlab/ci/rails.gitlab-ci.yml:db:check-schema:
.gitlab/ci/rails.gitlab-ci.yml:db:check-migrations:
.gitlab/ci/rails.gitlab-ci.yml:db:gitlabcom-database-testing:
.gitlab/ci/rails.gitlab-ci.yml:db:backup_and_restore:
.gitlab/ci/rails.gitlab-ci.yml:db:rollback geo:

Notes

You may find !67486 (closed) on how to make some of these jobs work with decomposed databases.