BREAKING fix(loki-mixin): rename rules key (#691)

Renames the `prometheus_rules` key to `prometheusRules` to comply to the
spec (https://github.com/monitoring-mixins/docs/blob/master/design.pdf)

BREAKING `prometheus_rules` is not available anymore

production/loki-mixin/recording_rules.libsonnet

 local utils = import "mixin-utils/utils.libsonnet";
 
 {
-  prometheus_rules+:: {
+  prometheusRules+:: {
     groups+: [{
       name: 'loki_rules',
       rules: