This commit upgrades the SonarQube plugin so that it is compatible with
Gradle 3.

Issue: SPR-14569

This commit also sets the test source encoding to UTF-8.

Issue: SPR-14547

* reactor-core 3.0.1.BUILD-SNAPSHOT
* reactor-netty 0.6.0.BUILD-SNAPSHOT

This commit ensures that JUnit XML reports are generated in the old
Gradle 2.x location for all tests in the spring-test module.

Issue: SPR-14569

Issue: SPR-14569

Since the upgrade to Gradle 3, our Bamboo build is failing because it
can't find test reports "at the usual location".

This commit restores the location that Gradle 2 was using until we
upgrade to a version of Bamboo that supports it.

Issue: SPR-14569

Previously the IDE (STS) was resolving a different reactor-core
version (3.0.0.BUILD-SNAPSHOT) than the build (3.0.0.RELEASE). This
discrepancy was due to the fact that reactor-netty now brings in
reactor-core 3.0.0.RELEASE. Gradle's version conflict resolution is to use
the latest version. However, the optional dependency on reactor-core
3.0.0.BUILD-SNAPSHOT was making the build path of the IDE use the SNAPSHOT.

This fix leverages a resolutionStrategy to ensure a consistent version of
reactor-core is used throughout the entire project. It also bumps up the
reactor-core version to 3.0.0.RELEASE to be consistent with reactor-netty.
Care is taken to only change 3.x vesion since 2.x is still used within
the codebase.

Issue: SPR-14569

Issue: SPR-14328

Issue: SPR-13344

Issue: SPR-13344

This commit also drops the now-unused CUSTOM_COMPILATION test group and the outdated JavaVersion class.

Issue: SPR-10423

Issue: SPR-11569

Issue: SPR-13575

Some reactor modules such as reactor-netty were migrated under a new
namespace `io.projectreactor.ipc`.

xmlunit 2.1.0 is the latest release for xmlunit.
Most of the xmlunit functionality used within spring-framework
was done through the xmlunit 1.x helper class
`org.custommonkey.xmlunit.XMLAssert`.

As of xmlunit 2.0.0 most of the XML comparison methods are done
through hamcrest matchers exposed by the xmlunit-matchers
library. In some cases during the migration, the matchers
had to be customized with custom `NodeMatcher` or
`DifferenceEvaluator` instances in order to keep the assertions
correct (they were performed with xmlunit 1.x previously).

Issue: SPR-14043

This commit adds support for Google Protobuf 3.0.0 and make some changes
in the additional formats support:
* "com.googlecode.protobuf-java-format:protobuf-java-format" is no
longer required and its required version has been raised to 1.3+
(this lib adds support for JSON, XML, HTML formats)
* "com.google.protobuf:protobuf-java-util" is also now supported for
JSON format

Issue: SPR-13589

Issue: SPR-13344

Issue: SPR-14467

 - Added Woodstox before Aalto in the spring-web build, so that Woodstox
   is used as StAX implementation, and not the less featured Aalto.
 - Hardcoded Aalto dependency in XmlEventDecoder, instead of relying on
   the StAX XMLInputFactory.