chore: Specify C# version #831

Job	Run time
test-fake-database	21s
test-server	22s
framework (random, no-auth)	10m 19s
framework (controlled, no-auth)	10m 54s
core (random, all-features)	37m 23s
core (random, single-seller)	35m 50s
core (random, no-payment-reconciliation)	36m 45s
core (random, no-auth)	35m 58s
core (random, no-tax-calculation)	35m 42s
core (random, prepayment-always-required)	34m 58s
core (random, facilityuse-has-slots)	36m 28s
core (controlled, all-features)	38m 40s
core (controlled, single-seller)	38m 14s
core (controlled, no-payment-reconciliation)	37m 43s
core (controlled, no-auth)	38m 10s
core (controlled, no-tax-calculation)	36m 52s
core (controlled, prepayment-always-required)	36m 53s
core (controlled, facilityuse-has-slots)	38m 0s
deploy-reference-implementation	0s
publish-server	0s
publish-fake-database	0s
	8h 59m 32s

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