As of Spring Framework 5.0, only Servlet 3.1+ containers are supported.
See https://github.com/spring-projects/spring-framework/wiki/Upgrading-to-Spring-Framework-5.x

This commit updates the reference documentation to reflect that and also
removes sections describing pre-Servlet 3.1 solutions, as those are not
supported anymore.

Fixes gh-12370

Even though War deployment is supported by Spring Framework itself for
WebFlux applications, this is not the case currently in Spring Boot.

Fixes gh-12455

* pr/12495:
  Use ApplicationContextRunner in tests

Closes gh-12495

Closes gh-12503

Closes gh-12502

Closes gh-12501

Closes gh-12500

Closes gh-12499

Closes gh-12393

* pr/12492:
  Fix Javadoc for ServiceLevelAgreementBoundary

Closes gh-12498

Closes gh-12497

Closes gh-12496

Closes gh-12492

This commit removes the required dependency on `WebClientCustomizer`
instances when creating `WebClient` beans, making it optional.

Closes gh-12458

Update `JettyReactiveWebServerFactory`/`TomcatReactiveWebServerFactory`
to explicitly set `asyncSupported` to `true`.

Fixes gh-12486

* pr/12472:
  avoid check class by string name

* pr/12459:
  Add missing super() calls

Closes gh-12459

This commit fixes 220f8cdc and moves the order for
`WelcomePageHandlerMapping` to `2` since the previous order was
conflicting with the resource mapping.

Closes gh-12335

Closes gh-12480

* pr/12477:
  Polish "Move test to ApplicationContextRunner"
  Move test to ApplicationContextRunner

Closes gh-12477

Move `DispatcherServletAutoConfigurationTests` to use the
`ApplicationContextRunner`

See gh-12477

* pr/12475:
  Replace Mockito.times(0) with Mockito.never()

Closes gh-12475

* atelechev/refactor-spring-boot-json-parsers:
  Polish "refactor spring-boot JSON parser"
  Refactor spring-boot JSON parsers

Polish JSON parser refactoring and remove some more duplication.

Closes gh-12428

Refactor JSON parser wrappers to remove duplicate code portions in the
parseMap() and parseList() methods by adding an AbstractJsonParser.

See gh-12428

This commit orders the `WelcomePageHandlerMapping` at
`Ordered.LOWEST_PRECEDENCE -1` in order to give a chance to other
mappings to handle the incoming requests.

In this case, developers might provide a custom `ViewController` or
custom `HandlerMapping` for the `"/"` path and we should not override
that opinion.

Closes gh-12335

This reverts commit 282bd9f0

This reverts commit 26d9c261

This commit updates WebTestClientAutoConfiguration to only create a
WebTestClient when running a WebFlux-based application as mocking the
context only works with that mode at the moment.

Closes gh-12318

This commit updates Spring Boot's DeferredImportSelector implementations
to group imports in a consistent set. This makes sure ordering is
applied consistently.

Closes gh-12366