Closes gh-23772

Closes gh-23910

Closes gh-23769

Closes gh-23792

Closes gh-23862

Closes gh-23861

Closes gh-23860

Closes gh-23859

Closes gh-23858

Closes gh-23857

Closes gh-23856

Closes gh-23855

Closes gh-23854

Closes gh-23853

Closes gh-23852

Closes gh-23851

Closes gh-23850

Closes gh-23849

Closes gh-23848

Closes gh-23847

Closes gh-23846

Closes gh-23845

Closes gh-23844

Closes gh-23843

Closes gh-23842

Closes gh-23841

Closes gh-23840

Closes gh-23839

Closes gh-23838

Closes gh-23837

Closes gh-23836

Closes gh-23835

Closes gh-23834

Closes gh-23833

Closes gh-23832

Prior to this commit, the how-to documentation would say that Spring
Boot does not support the h2c protocol. While it's not supported
out-of-the-box with a configuration property, this protocol can still be
configured using server customizers.

This commit documents, with code snippets, the server customizers one
should use to configure the h2c protocol in an application - for each
supported server.

Closes gh-21997

Prior to this commit, tests using the `@WebMvcTest` annotation would not
include the `HttpEncodingAutoConfiguration`. This means that, even if
configured, the encoding filter would not be configured in MVC tests,
resulting in an inconsistency with `@SpringBootTest` tests.

This commit ensures that the `HttpEncodingAutoConfiguration` is included
when `@WebMvcTest` is used.

Fixes gh-23749