Revert "Merge pull request #4119 from eXist-db/dependabot/maven/log4j.version-2.16.0"

This reverts commit a2eaa45, reversing
changes made to 6828b0e.

Author: adamretter

exist-parent/pom.xml

@@ -111,7 +111,7 @@
         <jaxb.api.version>3.0.1</jaxb.api.version>
         <jaxb.impl.version>3.0.2</jaxb.impl.version>
         <jetty.version>9.4.44.v20210927</jetty.version>
-        <log4j.version>2.16.0</log4j.version>
+        <log4j.version>2.15.0</log4j.version>
         <lucene.version>4.10.4</lucene.version>
         <milton.version>1.8.1.3</milton.version>
         <saxon.version>9.9.1-8</saxon.version>