update CHANGELOG.md to include #6603

Author: katbyte

CHANGELOG.md

@@ -12,6 +12,7 @@ BUG FIXES:
 * `azurerm_sql_database` - prevent extended auditing policy for secondary databases [GH-6402]
 * `azurerm_web_application_firewall_policy` - support for the `managed_rules` property which is required by the new API version [GH-6126]
 * `azurerm_virtual_network_gateway_connection` - `shared_key` is now optional when `type` is `IPSec` [GH-6565]
+* `azurerm_monitor_diagnostic_setting` - make `retention_policy` and `retention_policy` optional [GH-6603]
 
 ## 2.7.0 (April 23, 2020)