Deprecate the Prometheus SLI service

[jetzlstorfer]

As the Prometheus SLI service and Prometheus service are now merged (thanks to @rajdas98) we need to deprecate this service as its functionality is fully moved in the Prometheus service.

Proposed approach:

• [x] Announce in the README that service will be deprecated - @jetzlstorfer
• [x] Update documentation on Keptn.sh - @rajdas98
• [ ] Update integrations page on Keptn.sh - @johannes-b
• [ ] Update tutorials for Keptn 0.8 - @rajdas98
• [ ] After 30 days make the repo read-only
• [ ] Double check that issues/PRs are disabled

[imrajdas]

Hi @jetzlstorfer , Can you assign me?