Update log4j version to 2.15.0

This is to safeguard users of java-spdx-library against a vulnerability
(CVE-2021-44228) which could potentially allow remote code execution.

Signed-off-by: Sebastian Crane <[email protected]>

Author: seabass-labrax

pom.xml

@@ -93,7 +93,7 @@
     <dependency>
     	<groupId>org.apache.logging.log4j</groupId>
     	<artifactId>log4j-slf4j-impl</artifactId>
-    	<version>2.10.0</version>
+    	<version>2.15.0</version>
     </dependency>
     <dependency>
     	<groupId>org.apache.commons</groupId>