See gh-22959

Closes gh-22994

Closes gh-22987

Closes gh-22983

Closes gh-22982

ViewResolutionResultHandler no longer adds BindingResult objects for
async model attributes. Instead AbstractView adds them later when
those attributes are resolved to concrete values.

Closes gh-22933

Closes gh-22976

Closes gh-22975

Closes gh-22952

Closes gh-22960

See gh-22932

Closes gh-22955

See gh-22932

Prior to this commit, HeaderResultMatchers had a direct dependency on
org.junit.Assert which forces JUnit 4 to be present on the classpath.

This commit fixes this by introducing assertNotNull() in
org.springframework.test.util.AssertionErrors and delegating to that
instead.

Fixes gh-22932

* pr/22941:
  Fix typo in package-info.java

Closes gh-22941

* pr/22938:
  Fix typo

Closes gh-22938

Closes gh-22936

Closes gh-22931

Closes gh-22506

This commit adds proper reference counting to the UndertowDataBuffer.

Closes gh-22912

Closes gh-22903

See gh-22900

See gh-22900

See gh-22900

Closes gh-22896

Closes gh-22747