provider/aws: Set aws_subnet ipv6_cidr_block to computed (#14542)

Fixes: #14361

An IPv6 CIDR block is option and can be added *after* a subnet has been
created. Therefore, we should set it to `Computed: true`

Otherwise, a manually created IPv6 association will be removed on the
next terraform run

builtin/providers/aws/resource_aws_subnet.go

@@ -41,6 +41,7 @@ func resourceAwsSubnet() *schema.Resource {
 			"ipv6_cidr_block": {
 				Type:     schema.TypeString,
 				Optional: true,
+				Computed: true,
 			},
 
 			"availability_zone": {