Rename Default Roles to Predefined Roles

Rename Default Roles to Predefined Roles

The term 'default roles' wasn't quite apt as these roles aren't able to
be modified or removed after installation, so rename them to be
'Predefined Roles' instead, adding an entry into the newly added
Obsolete Appendix to help users of current releases find the new
documentation.

Bruce Momjian and Stephen Frost

Discussion: https://postgr.es/m/157742545062.1149.11052653770497832538%40wrigleys.postgresql.org
and https://www.postgresql.org/message-id/20201120211304.GG16415@tamriel.snowman.net