Add job that upgrades common and creates a MR

Update the CI configuration and add 2 jobs:

• a job that upgrades the common dependency in a new branch, pushes that branch, and creates a MR
• a job that always fails, to make sure the rest of the pipeline is skipped

These jobs belong to a new maintenance stage that runs before any other stage. They are created only when the MAINTENANCE variable is set.

See update_analyzers job added to the pipeline of the common library in gitlab-org/security-products/analyzers/common!89 (merged).

The CHANGELOG.md is not updated, so this is to be used when the upgrade of the common doesn't warrant a changelog entry.

See gitlab-org/gitlab#11466 (comment 322828213)