Use update-prometheus-3.0 branch #15908

Job	Run time
Go build with -tags=stringlabels	46s
Check misspelled words	14s
Go build for different platforms	9m 48s
Thanos end-to-end tests (8, 0)	4m 58s
Linters (Static Analysis) for Go	2m 43s
Thanos unit tests	9m 19s
Thanos end-to-end tests (8, 1)	3m 52s
Thanos end-to-end tests (8, 2)	3m 55s
Thanos end-to-end tests (8, 3)	4m 14s
Thanos end-to-end tests (8, 4)	4m 23s
Thanos end-to-end tests (8, 5)	4m 11s
Thanos end-to-end tests (8, 6)	4m 4s
Thanos end-to-end tests (8, 7)	3m 38s
	56m 5s

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