Run webflux's tests in parallel to shorten long tail of builds (#23701)

When built in an environment where many Gradle build workers are
available, :spring-webflux:test is regularly the only task running at
the end of the build. Therefore, the build's overall execution time
can be reduced by running its tests in parallel, spreading the tests'
execution across the available workers. The configured number of forks
is a maximum with Gradle reducing this as necessary for environments
with low numbers of cores where multiple workers will not improve
build performance.

Closes gh-23701

spring-webflux/spring-webflux.gradle

@@ -55,3 +55,7 @@ dependencies {
 	testRuntime("com.sun.xml.bind:jaxb-impl")
 	testRuntime("com.sun.activation:javax.activation")
 }
+
+test {
+	maxParallelForks = 4
+}