Deprecate support for MongoDB 4.0 and 4.2

Debezium mainline currently supports the following versions of MongoDB:

However, per MongoDB Software Lifecycle Schedules 4.0 is EOL and 4.2 will be EOL in April:

Since it takes 2 major releases to deprecate and remove support, we should begin the deprecation process ASAP so that we can remove the constraints put on Debezium in the implementation space.