chore: Specify C# version #828

Job	Run time
test-fake-database	45s
test-server	19s
framework (random, no-auth)	2m 56s
framework (controlled, no-auth)	4m 51s
core (random, all-features)	37m 39s
core (random, single-seller)	35m 58s
core (random, no-payment-reconciliation)	36m 20s
core (random, no-auth)	36m 13s
core (random, no-tax-calculation)	36m 19s
core (random, prepayment-always-required)	34m 36s
core (random, facilityuse-has-slots)	36m 19s
core (controlled, all-features)	38m 31s
core (controlled, single-seller)	38m 57s
core (controlled, no-payment-reconciliation)	37m 33s
core (controlled, no-auth)	38m 4s
core (controlled, no-tax-calculation)	37m 21s
core (controlled, prepayment-always-required)	36m 48s
core (controlled, facilityuse-has-slots)	39m 30s
deploy-reference-implementation	0s
publish-server	0s
publish-fake-database	0s
	8h 48m 59s

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