Remove module/features in `3.x`

[ppkarwasz]

We should remove the following Maven modules or features 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.

[ppkarwasz]

The following modules are not deprecated, but they should be removed from the main repo (cf. #1966):

• log4j-jakarta-smtp,
• log4j-jakarta-web,
• log4j-jms,
• log4j-smtp,
• log4j-web.

[vy]

This issue is addressed by #1981 and #1985.