This commit also aligns ResourceUrlProvider's and RequestMappingInfo's UrlPathHelper setter/getter signatures.

Issue: SPR-14454

A "What's New" page has been created in the Spring Framework wiki:

https://github.com/spring-projects/spring-framework/wiki/What's-New-in-the-Spring-Framework

This commit therefore links to the new wiki page from the appendix of
the reference manual and deletes the existing "What's New" chapter from
the reference manual.

Issue: SPR-14449

This commit makes use of the new Supplier<String> variants of utility
methods in org.springframework.util.Assert within the spring-test
module.

Issue: SPR-14450

This commit makes use of the new Supplier<String> variants of utility
methods in org.springframework.util.Assert within the spring-core
module.

Issue: SPR-14450

Prior to this commit, utility methods in
org.springframework.util.Assert accepted String arguments for custom
error messages. Such Strings are evaluated (e.g., concatenated)
eagerly, and for performance reasons, it was therefore not always
possible to make use of these utility methods. Consequently, there are
several places in the code base that "inline" identical functionality
in order to lazily evaluate error message concatenation and avoid an
unnecessary performance penalty. This leads to verbose code like the
following.

if (!contextPath.startsWith("/")) {
    throw new IllegalArgumentException("contextPath '" + contextPath +
    "' must start with '/'.");
}
if (contextPath.endsWith("/")) {
    throw new IllegalArgumentException("contextPath '" + contextPath +
    "' must not end with '/'.");
}

This commit addresses this shortcoming by introducing Supplier<String>
variants of all utility methods in org.springframework.util.Assert that
allow custom error messages to be evaluated lazily via a lambda
expression that is only evaluated if the assertion fails. This results
in a simplification of the previous examples as follows.

Assert.isTrue(contextPath.startsWith("/"), () -> "contextPath '" +
    contextPath + "' must start with '/'.");
Assert.isTrue(!contextPath.endsWith("/"), () -> "contextPath '" +
    contextPath + "' must not end with '/'.");

Issue: SPR-14450

Issue: SPR-14247

* pr/1105:
  Polish doc

Closes gh-1105

* pr/1107:
  Polish doc

Closes gh-1107

Issue: SPR-13794

This commit introduces 'before' and 'after' test execution callbacks in
the Spring TestContext Framework. Specifically, this set of commits
introduces the following.

- beforeTestExecution() and afterTestExecution() callbacks in the
  TestExecutionListener API

- beforeTestExecution() and afterTestExecution() callbacks in
  TestContextManager

- RunBeforeTestExecutionCallbacks and RunAfterTestExecutionCallbacks
  JUnit 4 statements that are used by the SpringJUnit4ClassRunner

- Documentation in the class-level Javadoc for SpringMethodRule stating
  that before/after test execution callbacks cannot be supported with
  JUnit 4 Rules

- Support for JUnit Jupiter's BeforeTestExecutionCallback and
  AfterTestExecutionCallback extension APIs in the SpringExtension for
  JUnit 5

- Support for before/after test execution callbacks in
  AbstractTestNGSpringContextTests for TestNG

Issue: SPR-4365

Issue: SPR-4365

Issue: SPR-4365

Issue: SPR-4365

Issue: SPR-4365

Issue: SPR-4365

This commit fixes the method that takes a Callable to actually always
invoke it rather than returning null.

Issue: SPR-14445

This commit fixes a minor bug introduced in 0adc4921.

Issue: SPR-14447

Issue: SPR-14430

Issue: SPR-14432

Issue: SPR-13188

Issue: SPR-14432

Issue: SPR-13794

Issue: SPR-14436

Issue: SPR-13188

Issue: SPR-14427

This commit removes `GuavaCache` and support classes. Caffeine supersedes
the caching support in the Google Guava library with an actively maintained
Java 8+ version in standalone form.

As it is the only Guava feature Spring framework integrates with, this
commit removes effectively any reference to Guava.

Issue: SPR-13797

SPR-14055 introduced first-class support for the Java 8 Parameter API.

This commit therefore replaces the MethodParameterFactory with use of
the new SynthesizingMethodParameter.forParameter(Parameter) factory
method.

Issue: SPR-13575