feat: Increase hard slo for elastic single nodes to 95%.

Part of remediation for https://gitlab.com/gitlab-com/gl-infra/observability/team/-/issues/3997#note_2340617597

The high water mark for Elastic is set to 97% or 98% depending on the node type, so this doesn't put us at additional risk, just gives us a little less time to respond if necessary.

Once we've calmed down the warm nodes, we'll drop this back.