Upgrade Loki Operator to use Loki v2.7.3:

• Create upstream-v2.7.3 branch on openshift/loki
• Add OWNERS file to upstream-v2.7.3
• Add OCP-based Dockerfiles for Loki and Promtail in upstream-v2.7.3
• Ensure image mirroring of v2.7.3 on quay.io/openshift/logging for Loki and Promtail
• Switch Operator references to use v2.7.3
• Switch OpenShift Logging 5.6 and 5.7 midstreams to use branch upstream-v2.7.3 for Loki
• Sync upstream grafana/loki main into openshift/loki main

Developer Notes

1. Link all Pull and Merge requests for accomplishing the upgrades in this single task and NOT on the subtasks.