* pr/13320:
  Polish

Closes gh-13320

Fixes gh-13122

Closes gh-13318

This commit separates the auto-configuration of JavaMailSender in two
distinct configuration: one based on existing Session available on JNDI
and the other one based on properties configuration.

This clarifies the auto-configuration report and the fact that the JNDI
variant overrides any Session-related properties that would have been
set in the environment

Closes gh-13026

Closes gh-13314

* pr/13308:
  Remove outdated reference to @EnableReactor

Closes gh-13308

Add ExpectedException rule which was missed from the 1.5.x merge.

Update `JarURLConnection.get()` to guard against the wrong nested JAR
being used as context.

Closes gh-11367

Closes gh-13307

Closes gh-13142

Update `ErrorPageFilter` so that a Tomcat `ClientAbortException` no
longer causes "Cannot forward to error page for request" logging for
committed responses. Since a `ClientAbortException` indicates that the
client is no longer available it's of no consequence that the request
has been committed and the forward will fail.

Closes gh-13221

Closes gh-13291