chore: prepare for v0.15.6-rc.1 release#1864

bwplotka merged 1 commit intorelease/0.15GoogleCloudPlatform/prometheus-engine:release/0.15from

ridwanmsharif/0.15-choreGoogleCloudPlatform/prometheus-engine:ridwanmsharif/0.15-choreCopy head branch name to clipboard