chore(ci): test with latest and next showcase images

[kadel]

Description of the change

• Automated image builds backstage-showcase#545 got fixed, so we can revert to using the regular latest tag
• this also adds new test using next tag to validate that this chart is compatible with the next release (main branch)

Existing or Associated Issue(s)

Additional Information

Checklist

• Chart version bumped in Chart.yaml according to semver.
• Variables are documented in the values.yaml and added to the README.md. The pre-commit utility can be used to generate the necessary content. Use pre-commit run -a to apply changes.
• JSON Schema template updated and re-generated the raw schema via pre-commit hook.
• List tests pass for Chart using the Chart Testing tool and the ct lint command.

[kadel]

@tumido, what do you think about this?

[tumido]

@kadel , do we want to potentially run all tests against both images or is this just one test case?

If it is just one test case, we should define it as one instead by adding a values file into this folder:

https://github.com/janus-idp/helm-backstage/tree/main/charts/backstage/ci

[kadel]

All tests against both images. I want to make sure that updates to this helm chart are not somehow incompatible with Janus current and future releases.

[tumido]

/lgtm
/approve