Closes gh-9828

Closes gh-9620

* gh-9678:
  Polish "Deprecate JsonContent.assertThat()"
  Deprecate JsonContent.assertThat()

Closes gh-9678

See gh-9678

Previously, background preinitialization was started in response to
an ApplicationEnvironmentPreparedEvent and would complete at an
undetermined time later. This opened a window where SpringApplication
run could return and background preinitialization could still be
in progress. If, within this window, something attempted to configure
the logging system, an IO failure could occur as logging on the
background preinitialization thread would attempt to use resources
that had been closed.

This commit updates BackgroundPreinitializer so that it waits for
preinitialization to have completed when it receives an application
ready or application failed event. This prevents SpringApplication
run from returning while preinitialization is still in progress,
closing the window described above.

With info level logging enabled it appears that background
preinitialization consistently completes before the application ready
event is published. As a result, waiting should have no adverse effect
on performance in normal circumstances. With logging configured such
that background preinitialization outputs a large volume of log
messages (enabling trace logging for the root logger, for example), it
will be slowed down sufficiently for waiting to be necessary.

Closes gh-5669

Background preinitialization triggers static initialization of a
number of components that are slow to initialize. As the
initialization is static, it's only necessary once per class loader.

Previously, a new background preinitialization thread would be
created and started for each ApplicationEnvironmentPreparedEvent.
This commit updates the preinitializer to only create and start the
thread if preinitialization has not already been started for the
current class loader.

Closes gh-9869

This commit ensures that an exception that is thrown as part of the
`ContextConsumer` callback is thrown as is.

Closes gh-9878

Closes gh-9871

Closes gh-9875

Update `ApplicationContextRunner` so that it is totally immutable.
Methods now return new instances rather than changing existing state.

See gh-9875

Update `TestPropertyValues` so that it is totally immutable. Methods
now return a new instance rather than changing existing state.

See gh-9875

Split up `org.springframework.boot.test.context` into distinct packages
for `runner` and `assertj`.

See gh-9875

Rename `ApplicationContextTester` and related classes to
`ApplicationContextRunner` and refactor existing tests to use correctly
named variables.

See gh-9875

* pr/9763:
  Polish "Polish Hibernate naming strategy doc"
  Polish Hibernate naming strategy doc

Closes gh-9763

See gh-9763

* pr/9855:
  Polish

Closes gh-9855

See gh-9855

This commit adds the `jackson-module-kotlin` dependency to the JSON
starter. As the other modules selected there, the goal is to provide
additional Jackson support to Spring Boot projects.

In this case, the Kotlin language is supported by many Spring projects
and this module is essential for (de)serialization with Jackson.
Note that the module has a transitive dependency on `kotlin-reflect`,
but this dependency should be brought by the application itself.

Spring Framework will configure the Kotlin Jackson module *only if* the
Kotlin sdk is present.

Closes gh-9803

Closes gh-9767

Closes gh-9586

This commit resets the artificial use of the http driver now the bolt
driver doesn't check if the connection to the Neo4J server is valid
on startup.

See neo4j/neo4j-java-driver#380

Closes gh-9500

Closes gh-9834