Update serviceMonitor for Confluence

ServiceMonitor for Confluence fails when service port is set to integer. Here's the error with the current config:

warning: Upgrade "confluence" failed: failed to create resource: ServiceMonitor.monitoring.coreos.com "confluence" is invalid: spec.endpoints.port: Invalid value: "integer": spec.endpoints.port in body must be of type string: "integer"

Updated {{ .Values.confluence.service.port }} to {{ .Values.confluence.service.port | quote }} to resolve the issue.