Update dependency System.Linq.Async to v7 #7937

Job	Run time
containers / servicecontrol-monitoring	2m 7s
windows-installers / build	7m 40s
db-container / db-container	1m 12s
containers / servicecontrol-audit	2m 13s
containers / servicecontrol	2m 14s
Windows-AzureServiceBus	11m 56s
Windows-AzureStorageQueues	6m 9s
Windows-RabbitMQ	11m 13s
Linux-Default	11m 50s
Windows-PostgreSQL	6m 34s
Windows-SQS	6m 26s
Windows-Default	13m 38s
Windows-SqlServer	8m 44s
Linux-SqlServer	3m 48s
Windows-PrimaryRavenAcceptance	18m 5s
Linux-AzureServiceBus	9m 31s
Windows-PrimaryRavenPersistence	7m 52s
Windows-MSMQ	5m 5s
Linux-RabbitMQ	8m 16s
Linux-PrimaryRavenAcceptance	10m 1s
Linux-PostgreSQL	3m 38s
Linux-AzureStorageQueues	3m 51s
Linux-PrimaryRavenPersistence	4m 42s
Linux-SQS	3m 51s
container-test / asq	2m 50s
container-test / postgresql	2m 7s
container-test / mssql	1m 44s
container-test / sqs	2m 29s
container-test / asb	3m 22s
container-test / rabbit	2m 20s
	3h 5m 28s

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