Fix support for keep_firing_for field in alert rules #8544

Job	Run time
CodeQL	10m 18s
build	3m 54s
lint	3m 44s
test	20m 13s
integration-configs-db	2m 3s
integration (requires_docker)	5m 41s
integration (integration_alertmanager)	4m 9s
integration (integration_backward_compatibility)	8m 2s
integration (integration_memberlist)	7m 22s
integration (integration_querier)	12m 37s
integration (integration_ruler)	4m 5s
integration (integration_query_fuzz)	1m 54s
deploy	0s
deploy_website	0s
	1h 24m 2s

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