Map log levels for z/OS syslog

<!-- Please only use this template for submitting feature requests --> **Why is this needed**: To allow making sense of syslog from a prevalent mainframe operating system called z/OS. Plenty of enterprises use mainframes, and run many instances of z/OS. Having native support for log level formatting of z/OS syslog messages increases the chances of enterprise adoption of Grafana as a single pane of glass, across the estate. For context, a production z/OS system can produce anywhere from thousands up to hundreds of thousands of syslog lines per minute. Having the ability to at least get high level categorization of these messages in Grafana + Loki will be significantly helpful in making sense of this firehose. **What would you like to be ~~added~~ done**: For Grafana the company to accept a PR where I add the mapping of z/OS syslog message levels to the standard info, warn, error, etc. The initial change set is small, and only needs updates to: `pkg/services/featuremgmt/registry.go` `public/app/features/logs/utils.ts` `public/app/features/logs/utils.test.ts` I can follow up with nuanced log level mapping for some of the peculiar message types. **Who is this feature for?** Enterprises and Grafana customers who run mainframes, specifically z/OS systems. Note that plenty of Fortune x00 companies have critical workloads running on mainframes.