Bump log4j-core from 2.13.2 to 2.15.0

Bumps log4j-core from 2.13.2 to 2.15.0.

---
updated-dependencies:
- dependency-name: org.apache.logging.log4j:log4j-core
  dependency-type: direct:development
...

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Author: dependabot[bot]

pom.xml

@@ -50,7 +50,7 @@
     <slf4j.version>1.7.21</slf4j.version>
 
     <!-- For testing only. -->
-    <log4j.version>2.13.2</log4j.version>
+    <log4j.version>2.15.0</log4j.version>
     <hamcrest.optional>1.0</hamcrest.optional>
   </properties>