Merge Gitjob Controller into Fleet Controller / Chart

[manno]

rancher/gitjob is only used by Fleet, and adds to its maintenance/release overhead, both for Go code and for charts.
Instead, we could integrate rancher/gitjob contents into rancher/fleet and manage all Fleet-related logic from there.

Note:
Merging gitjob into the same code base adds some interesting possibilities in the future. We might be able to remove the gitjob CRD, the gitjob container image, so we have less complexity and a smaller download size.
This will become clearer when be build OCI repo support, but gitjob/gitrepo are as much their own component as oci repo is. So the split between gitjob vs gitrepo doesn’t make sense. Eventually, we might have “bundle” producing components (gitrepo, ocirepo) and a component which distributes bundles to clusters (fleetcontroller/targetting).
So, in short, we’re moving gitjob now, so maintenance and releases get easier, but it will also enable a clearer design in the future.

This would require us to:

• move tests
• remove the gitjob helm chart, placing it in the fleet chart (no more sub chart)
• move the gitjob controller into fleet, keep it in a separate pod (it polls an external resource)
• adapt drone workflow to publish image
• remove "subchart config" from rancher/charts a
• optional, move into same directory struct as fleet (internal/cmd/gitjob)
• can we get rid of the GitJob resource in the future and use GitRepo instead?
• keep the webhook in the same pod as gitjob

More housekeeping (in no particular order)

• remove updatecli job
• check .gitignore
• update references to rancher/gitjob in docs throughout rancher/fleet
• remove script .github/scripts/check-for-gitjob-changes.sh

[weyfonk]

Once #2098 is merged:

• Cherry-pick gitjob#421
• Cherry-pick gitjob#422

[weyfonk]

When next releasing Fleet from a branch including #2098:

• Remove packages/fleet/gitjob from rancher/charts' target branch (eg. by editing the PR generated by release-against-charts)
• Check if newly generated index.yaml entry in rancher/charts still includes gitjob as a chart; it should not.
• Check that packages/fleet/fleet/generated-changes no longer contains references to gitjob
• Check that the auto-generated release PR against rancher/charts does not add any docker.io/rancher/gitjob tag entry to regsync.yaml, but generates an entry for docker.io/rancher/fleet-gitjob instead.
• Test installing that Fleet release in Rancher.