Update SerializableProvider description in migration guide.

Change this text:

Deserializing Java objects from untrusted sources is not safe. For this reason, in JBoss EAP 7.1, the org.jboss.resteasy.plugins.providers.SerializableProvider class is disabled by default, and it is not recommended to use this provider.

to this text:

Deserializing Java objects from untrusted sources is not safe. For this reason, in JBoss EAP 7, the org.jboss.resteasy.plugins.providers.SerializableProvider class is disabled by default, and it is not recommended to use this provider.

This information about SerializableProvider needs to be also in EAP 7.0 migration guide.