Migrate all existing JUnit 4 `assert...` based assertions to AssertJ
and add a checkstyle rule to ensure they don't return.

See gh-23022

Migrate all existing `assertThat(..., Matcher)` assertions to AssertJ
and add checkstyle rules to ensure they don't return.

See gh-23022

Closes gh-23015

Closes gh-22943

Closes gh-22943

Closes gh-22959

Closes gh-23006

This commit introduces a getOrEmpty(String) method in HttpHeaders that
returns an immutable, empty list if no values are present for the
specified header name. This is provided as a convenience over the
existing get(String) method which returns null in such cases.

Closes gh-22949

Closes gh-23012

Aligns ReactiveAdapterRegistry with MVC/messaging handler methods in terms of recognizing CompletionStage as well as CompletableFuture. Includes consistent use of ReactiveAdapter for reactive transactions.

Closes gh-23011

This commit adds a leading slash for path predicates in both
WebFlux.fn and WebMvc.fn.

Closes gh-22795

See gh-22959

Migrate tests that use `@test(expectedException=...)` or
`try...fail...catch` to use AssertJ's `assertThatException`
instead.

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