Document module deprecation in 2.x

Since layout deprecation was already done in #2527, we
document:

* The deprecation of some appenders, pointing to third-party
alternatives, when possible.
* The deprecation of the Log4j Tag library.

The Spring Boot Lookup and Java EE SMTP Appender will
disappear in version 3.x, but will be replaced with the
Spring Boot 3 Lookup (in Spring Boot) and Jakarta EE
SMTP Appender.

Co-authored-by: Volkan Yazıcı <volkan@yazi.ci>
4 files changed