improve autogen rendering for PG ExcludeConstraint

improve autogen rendering for PG ExcludeConstraint

Fixed issue regarding PostgreSQL :class:`.ExcludeConstraint`, where
constraint elements which made use of :func:`.literal_column` could not be
rendered for autogenerate. Additionally, using SQLAlchemy 2.0.5 or greater,
:func:`.text()` constructs are also supported within PostgreSQL
:class:`.ExcludeConstraint` objects for autogenerate render. Pull request
courtesy Jan Katins.

Fixes: #1184
Closes: #1185
Pull-request: https://github.com/sqlalchemy/alembic/pull/1185
Pull-request-sha: 68360ce9aa746a85407c5e1e04b8021123d98504

Change-Id: I302f3f4007a186ffac13b344ff6769dd302f28f5