Autodoc fails on PostgreSQL 12

[cbbrowne]

postgresql_autodoc -h localhost -u postgres -p 7012 -d mydb

when pointed at PostgreSQL 12

generates a bunch of messages of the form...

DBD::Pg::st execute failed: ERROR:  column "adsrc" does not exist
LINE 40:                 adsrc
                         ^ at /usr/local/bin/postgresql_autodoc line 695.

The cause is documented in the PostgreSQL 12 release notes:

Remove obsolete pg_attrdef.adsrc column (Peter Eisentraut)

This column has been deprecated for a long time, because it did not update in response to other catalog changes (such as column renamings). The recommended way to get a text version of a default-value expression from pg_attrdef is pg_get_expr(adbin, adrelid).

[cbbrowne]

FYI, I looked back as far as PostgreSQL 8.4; the above function exists that far back, so this should be nicely backwards compatible.