chore: Specify C# version #833

Job	Run time
test-fake-database	23s
test-server	20s
framework (random, no-auth)	42m 45s
framework (controlled, no-auth)	32m 45s
core (random, all-features)	37m 15s
core (random, single-seller)	36m 25s
core (random, no-payment-reconciliation)	35m 43s
core (random, no-auth)	36m 26s
core (random, no-tax-calculation)	35m 27s
core (random, prepayment-always-required)	35m 10s
core (random, facilityuse-has-slots)	37m 3s
core (controlled, all-features)	39m 13s
core (controlled, single-seller)	37m 38s
core (controlled, no-payment-reconciliation)	38m 32s
core (controlled, no-auth)	38m 28s
core (controlled, no-tax-calculation)	37m 28s
core (controlled, prepayment-always-required)	37m 27s
core (controlled, facilityuse-has-slots)	39m 7s
deploy-reference-implementation	0s
publish-server	0s
publish-fake-database	0s
	9h 57m 35s

Provide feedback