Remove auto-configuration support for Spring Session Mongo since it is
no longer supported in Spring Session 2.0.

See gh-9011

Closes gh-6013

`BasicErrorControllerIntegrationTest` is handling the context in each test
and yet it starts a useless test via `@SpringBootTest`. This commit
removes the useless runner and associated annotations.

* pr/8971:
  Polish "Do not expose `exception` error attribute by default"
  Do not expose `exception` error attribute by default

Closes gh-8971

See gh-8971

* pr/9002:
  Make Audit events MVC endpoint `after` parameter required

Closes gh-9002

* pr/8819:
  Add support for configuring Jetty's request log via the environment
  Add support for configuring Jetty's request log via the environment

Closes gh-8819

See gh-8819

Closes gh-8982

* pr/8568:
  Polish "Add reactive support for Spring Data Cassandra"
  Add reactive support for Spring Data Cassandra

Closes gh-8568

See gh-8568

Closes gh-8990

Now that reactor-netty has a strict set of transitive dependencies, this
commit refactors the managed dependencies to only list the required
ones. `netty-all` is still managed to avoid conflicts when pulled by
other libraries.

Fixes gh-8553

Closes gh-7612

Refine Mustache support to provide a cleaner separation between the
reactive and servlet implementations. The views have now moved to the
`spring-boot` project and the auto-configuration has been split into
two distinct `@Imports` to save needing full package declarations.

See gh-8941

Closes gh-8996

Closes gh-8995

Closes gh-8994

Closes gh-8993

Closes gh-8992

Closes gh-8991

This commit simplifies the Mustache support for Spring MVC and removes
the included (view-based) i18n support in favor of more idiomatic
constructs like Mustache lambdas.

Fixes gh-8941

This commit moves the existing Spring MVC Mustache support to its own
`servlet` package and adds a new one under `reactive` for the WebFlux
web applications.

New `MustacheView` and `MustacheViewResolver` types resolve and render
Mustache views for WebFlux applications.

Since this templating engine is now supported by two flavors of Spring
web apps, the `spring-boot-starter-mustache` does not depend anymore on
the `spring-boot-starter-web` one: it's up to the developer to add the
relevant starter `web` or `webflux` to their application.

Fixes gh-8648

Closes gh-8964

Excluded the commons-logging dependency pulled in by Spring Security
until they can provide a fix.

See gh-8985

* pr/8881:
  Polish "Add database initializer for Spring Integration"
  Add database initializer for Spring Integration