Add 6.0.0 upgrade guide for the deprecated require_ssl field. (#10833) (#18282)

[upstream:ebf018ad38d3905fa11e83c3d695b15e9b191d1d]

Signed-off-by: Modular Magician <[email protected]>

Author: modular-magician

website/docs/guides/version_6_upgrade.html.markdown

@@ -106,4 +106,10 @@ Description of the change and how users should adjust their configuration (if ne
 
 ### Resource-level change example header
 
-Description of the change and how users should adjust their configuration (if needed).
+Description of the change and how users should adjust their configuration (if needed).
+
+## Resource: `google_sql_database_instance`
+
+### `settings.ip_configuration.require_ssl` is now removed
+
+Removed in favor of field `settings.ip_configuration.ssl_mode`.