Add codeowners from STACKIT

[timebertt]

How to categorize this PR?

/area os open-source
/kind enhancement

What this PR does / why we need it:

At STACKIT, we offer flatcar as the primary OS for worker nodes. Hence, we rely on this extension.
As other interested parties faced out CoreOS/flatcar in their environments, we probably have the highest stakes in keeping this extension up-to-date.
To simplify things, this PR adds two codeowners from STACKIT (particular persons are not set in stone).
This allows us to formally assume responsibility for this repository.

TODO:

• Add repo/CI access for codeowners from STACKIT #97

Which issue(s) this PR fixes:

See https://gardener-cloud.slack.com/archives/C045DSWJZB9/p1711461148599709

Special notes for your reviewer:

I recall, that we cannot add contributors from outside SAP to the gardener org. With this, we cannot add STACKIT colleagues to the @gardener/gardener-extension-os-coreos-maintainers group.
If this has changed meanwhile, let me know and we can drop this PR and use the group instead.

Release note:

NONE

[gardener-robot-ci-1]

Thank you @timebertt for your contribution. Before I can start building your PR, a member of the organization must set the required label(s) {'reviewed/ok-to-test'}. Once started, you can check the build status in the PR checks section below.

[gardener-robot]

@timebertt Thank you for your contribution.

[timebertt]

/ok-to-test

[gardener-robot]

@timebertt You are not permitted to touch label reviewed/ok-to-test.

[timebertt]

/ok-to-test

[gardener-robot]

@timebertt You are not permitted to touch label reviewed/ok-to-test.

[timebertt]

@zkdev Now that @dergeberl and I are part of @gardener, we could also use the @gardener/gardener-extension-os-coreos-maintainers team instead of adding our usernames to CODEOWNERS. Then we could drop this PR.
WDYT?

[zkdev]

I prefer to explicitly mention you, next to core-os GitHub team, in CODEOWNERS file to avoid unintended side-effects.

[timebertt]

Ok, fine for me 👍

[timebertt]

/ok-to-test

[gardener-robot]

@timebertt You are not permitted to touch label reviewed/ok-to-test.

[timebertt]

@timebertt You are not permitted to touch label reviewed/ok-to-test.

Is this bound to the CODEOWNERS file? Or are we missing anything else?

[plkokanov]

/lgtm