feat: run one job concurrently to reduce chance of conflicts (#198)

Each run of releaser-pleaser acts on the same global state in the
forge. Therefore, parallel runs are unnecessary.

This commit also communicates to the GitHub and GitLab CI pipelines that
the releaser-pleaser jobs can be cancelled as early as possible.

- On GitHub Actions this can be guaranteed through the workflow
  settings. These settings are copied into each repository that uses
  releaser-pleaser, so users need to update this manually. I will add a
  note to the release notes for this.
- On GitLab CI/CD this requires the user to configure a project level setting to
  "auto-cancel redundant pipelines". We will not recommend user to set
  this, as it is quite invasive and can break their regular CI pipelines.

docs/tutorials/github.md

@@ -44,6 +44,10 @@ on:
       - labeled
       - unlabeled
 
+concurrency:
+  group: releaser-pleaser
+  cancel-in-progress: true
+
 jobs:
   releaser-pleaser:
     runs-on: ubuntu-latest