Remove unused imports
Avoid duplicate entries

Prior to this commit, the schema distribution archive contains
duplicate entries for the more recent XSD as it is reference both with
its version and as the "default" XSD (i.e. without a version).

This commit configures the task to apply a duplicatesStrategy so that
only the first entry remains.

Issue: SPR-12011
Add messaging module section

Issue: SPR-11847
Fix broken link
Custom cache annotations and AspectJ

This commit clarifies what should be done in order to use custom cache
annotations with AspectJ.

Issue: SPR-11503
Add more HttpHeaders constants for standard HTTP headers

Issue: SPR-12063