The system module did not define an ID at the root of the config, that made the V2 input loader only start the first journald input it saw because they both ended up with the same identifier (type, ID and path). This is fixed by defining an ID at the root of the configuration templates.

The journald input now also adds the input_id key to its loggers and a non-fatal error is now logged at debug level.

The system-logs input is now marked as experimental instead of stable.

Fix lint warnings by moving toJournalConfig to input_linux.go

Move TestSystemLogsCanUseLogInput to a file without the
linux build constraint so it can run on all OSes supported by the
system integration.

(cherry picked from commit b1c7478)

# Conflicts:
#	.buildkite/filebeat/filebeat-pipeline.yml

…-41381