Alerting: Fix rule storage to filter by group names using case-sensitive comparison (#88992)
* add test for the bug
* remove unused struct
* update db store to post process filters by group using go-lang's case-sensitive string comparison
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Co-authored-by: Alexander Weaver <weaver.alex.d@gmail.com>
# Conflicts:
# pkg/services/ngalert/store/alert_rule.go
# pkg/services/ngalert/store/alert_rule_test.go