Release Nebraska 3.1.0

[ervcz]

1. Preparation

• Update and merge CHANGELOG.md to add the released version

2. Release

• staging deployment works as expected (basic user actions, Flatcar update payload, etc.)
• Tag and push the released version (e.g: git tag -as 2.9.0)
• Wait for CI to be green; create a GitHub release associated to the release tag and use the CHANGELOG.md content to create the release body
• List contributors on the release note
• Bump both appVersion and version in Chart.yaml, commit and open a PR.
• Merge docs(nebraska): add Pocket ID OIDC setup guide flatcar-website#520
• Merge docs(nebraska): add multi-step updates guide flatcar-website#523

3. Announcements

• Brief version announcement in slack (k8s slack #flatcar) and the Flatcar Matrix channel