Have org.apache.logging.log4j.util.Base64Util invoke java.util.Base64…

[jluehe]

… directly instead of reflectively

[A clear and concise description of what the pull request is for along with a reference to the associated issue IDs, if they exist.]

Checklist

• Base your changes on 2.x branch if you are targeting Log4j 2; use main otherwise
• ./mvnw verify succeeds (if it fails due to code formatting issues reported by Spotless, simply run ./mvnw spotless:apply and retry)
• Non-trivial changes contain an entry file in the src/changelog/.2.x.x directory
• Tests for the changes are provided
• Commits are signed (optional, but highly recommended)