See gh-22466

Closes gh-22449

Closes gh-22444

Closes gh-22426

See: gh-12857

* pr/22463:
  Polish

Closes gh-22463

This commit moves mavenLocal() to the bottom of the repositories list
so that Gradle's Eclipse IDE support will properly link binary JARs to
their source JARS in generated Eclipse .classpath files.

According to Gradle experts, "`mavenLocal` should basically never be
the first repo in the list."

https://github.com/eclipse/buildship/issues/655#issuecomment-366227296

See: gh-456

This commit adds a tip clarifying that TestNG's @BeforeClass and
@BeforeSuite methods can not be annotated with Spring's @Transactional
annotation.

Closes: gh-456

Closes gh-22435

Prior to this commit, the negotiated content-type during the request
mapping phase would be kept as the response content-type header; this
information is used when rendering the error response and prevents a new
round of content negotiation to choose the media type that fits best.

This commit removes the response content type information at the
beginning of the error handling phase.

Fixes gh-22452

This commit is the first part of a more complete Coroutines
support coming in Spring Framework 5.2. It introduces suspendable
Kotlin extensions for Mono based methods in WebFlux classes like
WebClient, ServerRequest, ServerResponse as well as a Coroutines
router usable via `coRouter { }`.

Coroutines extensions use `await` prefix or `AndAwait` suffix,
and most are using names close to their Reactive counterparts,
except `exchange` in `WebClient.RequestHeadersSpec`
which translates to `awaitResponse`.

Upcoming expected changes are:
 - Leverage `Dispatchers.Unconfined` (Kotlin/kotlinx.coroutines#972)
 - Expose extensions for `Flux` based API (Kotlin/kotlinx.coroutines#254)
 - Introduce interop with `CoroutineContext` (Kotlin/kotlinx.coroutines#284)
 - Support Coroutines in `ReactiveAdapterRegistry`
 - Support Coroutines for WebFlux annotated controllers
 - Fix return type of Kotlin suspending functions (gh-21058)

See gh-19975

See gh-20945

Closes gh-22423

Closes gh-22412

Closes gh-22419