flatcar/flatcar-linux-update-operator v0.8.0 on GitHub

Added golangci-lint and codespell CI jobs using GitHub Actions.
Added CI for vendor directory, generated files, Go modules tidiness, Dockerfile etc.
Added missing documentation for all packages.
Added integration tests for pkg/updateengine.
Added unit tests for pkg/operator.

Updated used Go version to 1.16 and Alpine version for container images to 3.14.
Container images will be now published to ghcr.io/flatcar-linux/flatcar-linux-update-operator instead of
quay.io/kinvolk/flatcar-linux-update-operator.
update-agent now use a dedicated Service Account with lower privileges than the operator.
update-agent now use a dedicated Pod Security Policy.
update-operator now use more strict Pod Security Policy, as it no longer needs permissions
for agent related resources.
Flatcar Linux references has been changed to Flatcar Container Linux.
Migrated from glog to klog,
which is a recommended logger for Kubernetes operators.
pkg/drain has been integrated into pkg/k8sutil.
Errors returned from operator and agent runs are now properly wrapped and can be unwrapped by the user.
Remaining CoreOS references has been replaced with Flatcar.
pkg/k8sutil.GetVersionInfo() will now return an error when /etc/flatcar/update.conf file is present,
but not readable.
Marking node as Unschedulable should now be more robust on frequent Node object updates.
Renamed Go module from github.com/kinvolk/flatcar-linux-update-operator to github.com/flatcar-linux/flatcar-linux-update-operator, following the move of repository into new organization.

Fixed various typos found by codespell.
Operator will no longer leak leader election goroutine, it will be now shut down when stop channel gets closed.
Operator will now internally gracefully return an error when leader election is lost, so process can be restarted
to start a new leader election.

Removed no longer used Travis CI, Jenkins configuration files and build scripts.
Removed AttemptUpdate(), GetStatus() and RebootNeededSignal() functions from pkg/updateengine package.
Removed deprecated --manage-agent, --agent-image-repo and --analytics flags from update-operator.
Deprecated operator functionality to manage agent DaemonSet has been removed. Agent DaemonSet must be now deployed separately.