Bump log4j.version from 2.20.0 to 2.21.0 in /p3c-pmd

Bumps `log4j.version` from 2.20.0 to 2.21.0.

Updates `org.apache.logging.log4j:log4j-api` from 2.20.0 to 2.21.0

Updates `org.apache.logging.log4j:log4j-core` from 2.20.0 to 2.21.0

Updates `org.apache.logging.log4j:log4j-bom` from 2.20.0 to 2.21.0
- [Release notes](https://github.com/apache/logging-log4j2/releases)
- [Commits](apache/logging-log4j2@rel/2.20.0...rel/2.21.0)

---
updated-dependencies:
- dependency-name: org.apache.logging.log4j:log4j-api
  dependency-type: direct:production
  update-type: version-update:semver-minor
- dependency-name: org.apache.logging.log4j:log4j-core
  dependency-type: direct:production
  update-type: version-update:semver-minor
- dependency-name: org.apache.logging.log4j:log4j-bom
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

p3c-pmd/pom.xml

@@ -48,7 +48,7 @@
         <maven-surefire-plugin.version>3.1.2</maven-surefire-plugin.version>
         <animal-sniffer-maven-plugin.version>1.23</animal-sniffer-maven-plugin.version>
         <jacoco-maven-plugin.version>0.8.10</jacoco-maven-plugin.version>
-        <log4j.version>2.20.0</log4j.version>
+        <log4j.version>2.21.0</log4j.version>
         <slf4j.version>1.7.36</slf4j.version>
         <!--enforce-->
         <enforce.activate>true</enforce.activate>