Upgrade to Asciidoctor Gradle Plugin 4.0.2

[wilkinsona]

We're on 3.3.x at the moment. Just in case it causes problems, let's do it in 3.2.x to avoid the right of mangling the docs in next month's final 3.1.x release.

[wilkinsona]

This has caused some intermittent build failures due to asciidoctor/asciidoctor-gradle-plugin#721. We need to ensure that projects added to the asciidoctorExtentions configuration have been built before Asciidoctor runs. When they're not, the build may fail. For example, this will happen if spring-boot-testcontainers hasn't built its jar before spring-boot-docs:asciidoctor runs:

WARNING: Configuration property 'spring.testcontainers.beans.startup' not found.
Exception in thread "main" org.asciidoctor.gradle.remote.AsciidoctorRemoteExecutionException: ERROR: The following messages from AsciidoctorJ are treated as errors:
Configuration property 'spring.testcontainers.beans.startup' not found.
	at org.asciidoctor.gradle.remote.ExecutorBase.failOnWarnings(ExecutorBase.groovy:175)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:568)
	at org.codehaus.groovy.reflection.CachedMethod.invoke(CachedMethod.java:107)
	at groovy.lang.MetaMethod.doMethodInvoke(MetaMethod.java:323)
	at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:1268)
	at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:1035)
	at org.codehaus.groovy.runtime.InvokerHelper.invokePogoMethod(InvokerHelper.java:1029)
	at org.codehaus.groovy.runtime.InvokerHelper.invokeMethod(InvokerHelper.java:1012)
	at org.codehaus.groovy.runtime.InvokerHelper.invokeMethodSafe(InvokerHelper.java:101)
	at org.asciidoctor.gradle.remote.AsciidoctorJavaExec$_run_closure3.doCall(AsciidoctorJavaExec.groovy:76)
	at org.asciidoctor.gradle.remote.AsciidoctorJavaExec$_run_closure3.call(AsciidoctorJavaExec.groovy)
	at org.codehaus.groovy.runtime.DefaultGroovyMethods.each(DefaultGroovyMethods.java:2359)
	at org.codehaus.groovy.runtime.DefaultGroovyMethods.each(DefaultGroovyMethods.java:2344)
	at org.codehaus.groovy.runtime.DefaultGroovyMethods.each(DefaultGroovyMethods.java:2385)
	at org.asciidoctor.gradle.remote.AsciidoctorJavaExec.run(AsciidoctorJavaExec.groovy:68)
	at org.asciidoctor.gradle.remote.AsciidoctorJavaExec.main(AsciidoctorJavaExec.groovy:49)