Deprecate modules/features for removal in `3.x`

[ppkarwasz]

In order to concentrate our limited resources on the remaining modules,
the PMC decided to deprecate the following modules/features in 2.x and
to remove them in 3.x:

• log4j-cassandra,
• CouchDB appender,
• GELF appender,
• Kafka appender,
• jeroMQ appender,
• log4j-jpa,
• log4j-mongodb3,
• log4j-spring-boot,
• Java EE SMTP appender,
• log4j-taglib,
• JsonLayout,
• YamlLayout.

This issue consists of two tasks

• Document 2.x module/feature deprecation #1950
• Remove module/features in 3.x #1951