Externalize java-toolchain configuration

We should not use subprojects to perform configuration becaause it
does not allow for lazy loading and it can cause ordering problems.
In this case, the toolchain was not being used but instead it was
using the JAVA_HOME.

By splitting the configuration into a plugin and applying it to each
project it fixes the toolchain configuration

itest/context/spring-security-itest-context.gradle

@@ -1,4 +1,6 @@
 apply plugin: 'io.spring.convention.spring-test'
+apply plugin: 'java-toolchain'
+apply plugin: 'test-compile-target-jdk25'
 
 dependencies {
 	implementation platform(project(":spring-security-dependencies"))