chore: Specify C# version #816

Job	Run time
test-server	21s
test-fake-database	21s
framework (random, no-auth)	6m 18s
framework (controlled, no-auth)	2m 24s
core (random, all-features)	37m 11s
core (random, single-seller)	35m 48s
core (random, no-payment-reconciliation)	36m 10s
core (random, no-auth)	37m 15s
core (random, no-tax-calculation)	36m 52s
core (random, prepayment-always-required)	35m 22s
core (random, facilityuse-has-slots)	36m 34s
core (controlled, all-features)	38m 55s
core (controlled, single-seller)	37m 16s
core (controlled, no-payment-reconciliation)	38m 7s
core (controlled, no-auth)	38m 19s
core (controlled, no-tax-calculation)	37m 51s
core (controlled, prepayment-always-required)	38m 23s
core (controlled, facilityuse-has-slots)	39m 18s
publish-server	0s
publish-fake-database	0s
deploy-reference-implementation	0s
	8h 52m 45s

Provide feedback