Bump log4j-core from 2.13.3 to 2.15.0

Bumps log4j-core from 2.13.3 to 2.15.0.

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

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Author: dependabot[bot]

pom.xml

@@ -89,7 +89,7 @@
         <packageVersion.dir>com/staffbase/plugins/sdk</packageVersion.dir>
         <packageVersion.package>${project.groupId}.json</packageVersion.package>
         <jose4j.version>0.7.2</jose4j.version>
-        <log4j.version>2.13.3</log4j.version>
+        <log4j.version>2.15.0</log4j.version>
         <junit.version>4.13</junit.version>
         <mockito.version>3.5.2</mockito.version>
     </properties>