SpringBoot Upgrade to Asciidoctor Gradle Plugin 4.0.2

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.

Comment From: wilkinsona

This has caused some intermittent build failures due to https://github.com/asciidoctor/asciidoctor-gradle-plugin/issues/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)