Forwarded headers are now processed before ServerWebExchange is created
through ForwardedHeaderTransformer which has the same logic as the
ForwardedHeaderFilter but works on the request only.

ForwardedHeaderFilter is deprecated as of 5.1 but if registered it is
removed from the list of filters and ForwardedHeaderTransformer is used
instead.

Issue: SPR-17072

Issue: SPR-16989

Issue: SPR-17188

Issue: SPR-17117

Issue: SPR-17116

Issue: SPR-17115

Issue: SPR-17114

Issue: SPR-17100

Issue: SPR-17109

Includes consistent (non-)use of AnnotationUtils/AnnotatedElementUtils.

Issue: SPR-16933

Issue: SPR-17104

Issue: SPR-17032

This revision considers any java/javax and org.springframework.lang annotations as not synthesizable upfront, checking not only in isSynthesizable but also at synthesizeAnnotation(Array) level.

Issue: SPR-16933

Issue: SPR-17074

Includes revision of web.servlet.tags.form for non-null conventions.

Issue: SPR-15540

Empty Maps are preferably initialized without capacity (not initializing them at all or lazily initializing with default capacity when needed).

Issue: SPR-17105

Issue: SPR-16993

Obvious Fix

Issue: SPR-17090

Issue: SPR-16993

Issue: SPR-17091

Based on discussion under
https://github.com/spring-projects/spring-framework/pull/1901