Update v5 SDK tag for IPV6 firewall rules API (#17334)

Author: LeiWang3

specification/sql/resource-manager/readme.md

@@ -156,6 +156,7 @@ input-file:
 - ./Microsoft.Sql/preview/2021-05-01-preview/DistributedAvailabilityGroups.json
 - ./Microsoft.Sql/preview/2021-05-01-preview/ServerTrustCertificates.json
 - ./Microsoft.Sql/preview/2021-08-01-preview/ElasticPools.json
+- ./Microsoft.Sql/preview/2021-08-01-preview/IPv6FirewallRules.json
 
 
 # Needed when there is more than one input file