[release-5.6] Loki Operator ServiceMonitor relies on a BearerTokenFile, in violation with UWM Prometheus specification

Following the changes brought by [0], the Loki ServiceMonitor continues [1] to use the legacy `.spec.endpoints.bearertokensecret`, instead of the `.spec.endpoints.bearertokensecret` highlighted in [0]. 

The above is in direct violation with User Workload Monitoring (UWM) Prometheus specification, which triggers this [2] warning for every customer that relies on UWM in a cluster that has OpenShift Logging deployed as well.

[0] https://github.com/openshift/elasticsearch-operator/pull/903
[1] https://github.com/grafana/loki/blob/main/operator/bundle/openshift/manifests/loki-operator-metrics-monitor_monitoring.coreos.com_v1_servicemonitor.yaml#L14

[2]
~~~
level=warn ts=2023-11-09T15:21:33.009551274Z caller=operator.go:2255 component=prometheusoperator msg="skipping servicemonitor" error="it accesses file system via bearer token file which Prometheus specification prohibits" servicemonitor=openshift-operators-redhat/elasticsearch-operator-metrics-monitor namespace=openshift-user-workload-monitoring prometheus=user-workload
level=warn ts=2023-11-09T15:21:33.0096416Z caller=operator.go:2255 component=prometheusoperator msg="skipping servicemonitor" error="it accesses file system via bearer token file which Prometheus specification prohibits" servicemonitor=openshift-operators-redhat/loki-operator-metrics-monitor namespace=openshift-user-workload-monitoring prometheus=user-workload
~~~

UWM Prometheus triggers the above-highlighted warning for every customer that relies on UWM in a cluster that has OpenShift Logging deployed as well.

The Loki ServiceMonitor to be updated to use the `.spec.endpoints.bearertokensecret` specification, in compliance with the UWM Prometheus specification