chore: Specify C# version #815

Job	Run time
test-fake-database	21s
test-server	19s
framework (random, no-auth)	3m 41s
core (random, all-features)	36m 28s
framework (controlled, no-auth)	2m 53s
core (random, single-seller)	36m 2s
core (random, no-payment-reconciliation)	35m 58s
core (random, no-auth)	36m 24s
core (random, no-tax-calculation)	35m 20s
core (random, prepayment-always-required)	34m 43s
core (random, facilityuse-has-slots)	35m 58s
core (controlled, all-features)	39m 30s
core (controlled, single-seller)	37m 40s
core (controlled, no-payment-reconciliation)	38m 33s
core (controlled, no-auth)	38m 27s
core (controlled, no-tax-calculation)	37m 25s
core (controlled, prepayment-always-required)	36m 37s
core (controlled, facilityuse-has-slots)	39m 8s
publish-fake-database	0s
deploy-reference-implementation	0s
publish-server	0s
	8h 45m 27s

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