Bump webpack from 5.70.0 to 5.91.0 in /pkg/ui/react-app #13930

Job	Run time
Go build for different platforms	8m 58s
Thanos end-to-end tests (8, 0)	5m 18s
Linters (Static Analysis) for Go	1m 37s
Thanos end-to-end tests (8, 1)	3m 38s
Thanos end-to-end tests (8, 2)	3m 41s
Thanos end-to-end tests (8, 3)	4m 20s
Thanos end-to-end tests (8, 4)	7m 7s
Thanos end-to-end tests (8, 5)	4m 15s
Thanos end-to-end tests (8, 6)	4m 16s
Thanos end-to-end tests (8, 7)	3m 57s
	47m 7s

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