Separate PrometheusRules into multiple rule groups

[fourstepper]

Since mimir has the limit of ruler_max_rules_per_rule_group set to 20, we should either:

1. try splitting PrometheusRules into multiple rule groups for each SLO, somewhat logically
2. advise users to increase the ruler_max_rules_per_rule_group limit to a higher one

Cortex doesn't seem to be affected, as it, by default, doesn't impose any limits here.