SpringBoot Correct the version of Checkstyle that's used

https://github.com/spring-projects/spring-boot/issues/37976 should have upgraded the build to use Checkstyle 10.12.4 but, looking at the XML report files, we appear to be using 9.3:

<?xml version="1.0" encoding="UTF-8"?>
<checkstyle version="9.3">

Comment From: wilkinsona

Fixing this results in an NPE in one of our custom checks:

Caused by: java.lang.NullPointerException: Cannot invoke "io.spring.javaformat.checkstyle.check.SpringHeaderCheck$HeaderCheck.run(com.puppycrawl.tools.checkstyle.api.FileText, boolean)" because the return value of "io.spring.javaformat.checkstyle.check.SpringHeaderCheck.getCheck(java.io.File)" is null  
    at io.spring.javaformat.checkstyle.check.SpringHeaderCheck.processFiltered(SpringHeaderCheck.java:113)  
    at com.puppycrawl.tools.checkstyle.api.AbstractFileSetCheck.process(AbstractFileSetCheck.java:101)

This will have to be addressed in Spring Java Format.