chore: Specify C# version #823

Job	Run time
test-fake-database	43s
test-server	44s
framework (random, no-auth)	6m 25s
core (random, all-features)	37m 57s
framework (controlled, no-auth)	5m 16s
core (random, single-seller)	36m 36s
core (random, no-payment-reconciliation)	37m 31s
core (random, no-auth)	36m 58s
core (random, no-tax-calculation)	36m 37s
core (random, prepayment-always-required)	35m 49s
core (random, facilityuse-has-slots)	37m 7s
core (controlled, all-features)	39m 32s
core (controlled, single-seller)	37m 49s
core (controlled, no-payment-reconciliation)	38m 29s
core (controlled, no-auth)	38m 28s
core (controlled, no-tax-calculation)	37m 54s
core (controlled, prepayment-always-required)	37m 11s
core (controlled, facilityuse-has-slots)	39m 33s
deploy-reference-implementation	0s
publish-fake-database	0s
publish-server	0s
	9h 0m 39s

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