Bump xmlunit-core from 2.3.0 to 2.6.3

Bumps [xmlunit-core](https://github.com/xmlunit/xmlunit) from 2.3.0 to 2.6.3.
- [Release notes](https://github.com/xmlunit/xmlunit/releases)
- [Changelog](https://github.com/xmlunit/xmlunit/blob/master/RELEASE_NOTES.md)
- [Commits](xmlunit/xmlunit@v2.3.0...v2.6.3)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

build.gradle

@@ -28,7 +28,7 @@ dependencies {
     testImplementation('org.spockframework:spock-core:1.2-groovy-2.5') {
         exclude module: 'groovy-all' // use the version that is distributed with Gradle
     }
-    testImplementation 'org.xmlunit:xmlunit-core:2.3.0'
+    testImplementation 'org.xmlunit:xmlunit-core:2.6.3'
     testImplementation 'org.apache.commons:commons-text:1.6'
 }