chore: Specify C# version #829

Job	Run time
test-server	19s
test-fake-database	25s
framework (random, no-auth)	4m 27s
core (random, all-features)	37m 53s
framework (controlled, no-auth)	4m 59s
core (random, single-seller)	35m 11s
core (random, no-payment-reconciliation)	37m 4s
core (random, no-auth)	37m 19s
core (random, no-tax-calculation)	36m 28s
core (random, prepayment-always-required)	35m 32s
core (random, facilityuse-has-slots)	38m 32s
core (controlled, all-features)	39m 1s
core (controlled, single-seller)	37m 14s
core (controlled, no-payment-reconciliation)	38m 40s
core (controlled, no-auth)	38m 20s
core (controlled, no-tax-calculation)	37m 8s
core (controlled, prepayment-always-required)	36m 47s
core (controlled, facilityuse-has-slots)	38m 20s
deploy-reference-implementation	0s
publish-server	0s
publish-fake-database	0s
	8h 53m 39s

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