Add a note about setting GTID_MODE in Postgres Source for CDC

[grishick]

Tell us about the documentation you'd like us to add or update.

Some customers are running into the error documented in this issue. The error is caused by incompatible setting of GTID_MODE in their source database. It might make sense to add a note to the documentation of our Postgres Source connector, in CDC section that explains that Debezium only supports On and Off modes for GTID_MODE, but not ON_PERMISSIVE or any other modes.

If applicable, add links to the relevant docs that should be updated

• Link to relevant docs page