chore: Specify C# version #820

Job	Run time
test-server	21s
test-fake-database	23s
framework (random, no-auth)	2m 19s
framework (controlled, no-auth)	4m 12s
core (random, all-features)	38m 0s
core (random, single-seller)	37m 7s
core (random, no-payment-reconciliation)	36m 0s
core (random, no-auth)	36m 4s
core (random, no-tax-calculation)	36m 23s
core (random, prepayment-always-required)	35m 18s
core (random, facilityuse-has-slots)	36m 21s
core (controlled, all-features)	39m 34s
core (controlled, single-seller)	37m 28s
core (controlled, no-payment-reconciliation)	38m 30s
core (controlled, no-auth)	38m 43s
core (controlled, no-tax-calculation)	37m 29s
core (controlled, prepayment-always-required)	37m 10s
core (controlled, facilityuse-has-slots)	38m 55s
publish-server	0s
publish-fake-database	0s
deploy-reference-implementation	0s
	8h 50m 17s

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