changed: oidc username_claim default

[DEiselt]

What this PR does / why we need it: Changed default value of username_claim to preferred_username for use with Github.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged): Fixes #121

Special notes for your reviewer:

Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

TODOs:

• [ ] squash commits
• [ ] include documentation
• [ ] add unit tests