Note DBAPIs and dialects that we don't support

Note DBAPIs and dialects that we don't support

Since we have strong CI for the DBAPIs and dialects that are actively
supported, this indicates that those DBAPIs that aren't in CI are
continuing to fall behind in support, to the point where we can
not address issues that may arise. As such, the Sybase and Firebird
dialects overall are moving into an explicit "not supported" zone
where we would like to eventually remove them.   Additionally,
a pass is made through legacy MySQL and PostgreSQL DBAPI dialects
as well as those which we aren't able to include in CI to note
that these DBAPIs aren't actively supported by the project.

Change-Id: I61f1515b97b741b7534b54e434e3e47065df7b5d