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Fix the F# warnings of the build and make them errors #273

Job	Run time
Windows (.NET)	17m 44s
Linux (.NET) (build, SkipIntegrationTests=true)	2m 43s
Documentation	2m 11s
Linux (.NET) (flaky, PAKET_TESTSUITE_FLAKYTESTS=true)	3m 23s
Linux (.NET) (integration)	9m 8s
	35m 9s