This commit uses maven filtering to set the Groovy version rather than an
hard-coded value that can be easily outdated.

Closes gh-6530

Closes gh-6518

Closes gh-6499

This commit fixes the `@ConditionalOnClass` typo on the GsonTester
FactoryBean, which was testing for `ObjectMapper` instead of `Gson`.

This means that the next condition `@ConditionalOnBean(Gson.class)`
throws a `ClassNotFoundException: com.google.gson.Gson` since it's
trying to load that class to check for an existing Bean of that type.

Fixes gh-6502

Update spring-boot-docs so that an empty jar file is produced. The
code is only needed for documentation includes and if included in the
deployed jar seems to prevent promotion to Maven Central.

Fixes gh-6495

Closes gh-6318

Update ConfigurationPropertiesBindingPostProcessor with improved logging
when multiple PropertySourcesPlaceholderConfigurer beans are found.

See gh-6457

Replace `@UnmappedPropertyValue` with a `skip` attribute on
`@PropertyMapping` so that any default attribute value can be skipped.

Closes gh-6455

Closes gh-6490

* pr/6486:
  Add WireMock Spring Boot starter reference

Closes gh-6486

* gh-6487:
  Polish

Closes gh-6487

* pr/6461:
  Update Gradle remote debug documentation

Update the documentation to reference the `bootRun` command rather
than `run`.

Closes gh-6461

Update the reference documentation and add some additional Javadoc to
provide hints on how to customize the `TestRestTemplate`.

Fixes gh-6465

* patch-1:
  Unescape quotation marks in JavaDoc

Closes gh-6482

Switch `@AutoConfigureJsonTesters` to use regular `@Autowired` injection
for JSON testers. Prior to this commit JSON Tester fields were
initialized directly which caused IDE issues and was also a little
confusing.

Fixes gh-6451

Closes gh-6480

Closes gh-6481

Closes gh-6479

Closes gh-6478

Closes gh-6477

Closes gh-6476

Closes gh-6475

Closes gh-6474

Closes gh-6473

Closes gh-6472

Closes gh-6471