GHA migration part 2: Remove drone spec, drop Drone CI

[andreas-kupries]

Followup to #23, for rancher/rancher#44988

The PR #23 added the required GH action flows.
This PR now removes the Drone spec, dropping Drone CI.

[enrichman]

There are still a couple of Drone references:

• kube-api-auth/scripts/version

  Line 8 in 9814deb

  GIT_TAG=${DRONE_TAG:-$(git tag -l --contains HEAD | head -n 1)}

• kube-api-auth/Dockerfile.dapper

  Line 12 in 9814deb

  ENV DAPPER_ENV REPO TAG DRONE_TAG

Not sure about the Dapper one, but the first one will not work for sure. Are we using the version scripts somewhere in the CI?

[andreas-kupries]

There are still a couple of Drone references:

* https://github.com/rancher/kube-api-auth/blob/9814deb6ae4a9a2fdd0296eb771e743bbdfc0267/scripts/version#L8

* https://github.com/rancher/kube-api-auth/blob/9814deb6ae4a9a2fdd0296eb771e743bbdfc0267/Dockerfile.dapper#L12

We use scripts/version indirectly through scripts/build in the GHA flow.

The variable DRONE_TAG is one of the standard things set by the drone CI environment.
See https://docs.drone.io/pipeline/environment/reference/drone-tag/

While version tries to set GIT_TAG from DRONE_TAG, it does falls back to the git command when DRONE_TAG is empty or unset. That should be no trouble.

Further neither GIT_TAG nor VERSION are used in the flows.