Use pack stage for systemd journal logs

[james-callahan]

It's now generally accepted that log level shouldn't be a label; yet we currently capture it for systemd journal logs:

kustomize-loki/promtail/files/promtail.yaml

Lines 356 to 358 in 644e6b4

	- source_labels:
	- __journal_priority_keyword
	target_label: level

If we don't want to drop the log level as information, it looks like we should use the pack stage.
Potentially we could add other fields too (see man systemd.journal-fields)