Bump xmlunit-core from 2.6.3 to 2.6.4 (JabRef#6083)

Bumps [xmlunit-core](https://github.com/xmlunit/xmlunit) from 2.6.3 to 2.6.4.
- [Release notes](https://github.com/xmlunit/xmlunit/releases)
- [Changelog](https://github.com/xmlunit/xmlunit/blob/master/RELEASE_NOTES.md)
- [Commits](xmlunit/xmlunit@v2.6.3...v2.6.4)

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

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

build.gradle

@@ -206,7 +206,7 @@ dependencies {
     testRuntime group: 'org.apache.logging.log4j', name: 'log4j-core', version: '3.0.0-SNAPSHOT'
     testRuntime group: 'org.apache.logging.log4j', name: 'log4j-jul', version: '3.0.0-SNAPSHOT'
     testImplementation 'org.mockito:mockito-core:3.3.1'
-    testImplementation 'org.xmlunit:xmlunit-core:2.6.3'
+    testImplementation 'org.xmlunit:xmlunit-core:2.6.4'
     testImplementation 'org.xmlunit:xmlunit-matchers:2.6.4'
     testImplementation 'com.tngtech.archunit:archunit-junit5-api:0.13.1'
     testImplementation "org.testfx:testfx-core:4.0.17-alpha-SNAPSHOT"