maven: refactor usage of slf4j version (#3325)

Author: DaanHoogland

pom.xml

@@ -165,6 +165,7 @@
         <cs.xml-apis.version>1.4.01</cs.xml-apis.version>
         <cs.xmlrpc.version>3.1.3</cs.xmlrpc.version>
         <cs.xstream.version>1.4.10</cs.xstream.version>
+        <cs.slf4j.version>1.7.22</cs.slf4j.version>
         <org.springframework.version>5.0.2.RELEASE</org.springframework.version>
     </properties>
 
@@ -594,12 +595,12 @@
             <dependency>
                 <groupId>org.slf4j</groupId>
                 <artifactId>slf4j-api</artifactId>
-                <version>1.7.22</version>
+                <version>${cs.slf4j.version}</version>
             </dependency>
             <dependency>
                 <groupId>org.slf4j</groupId>
                 <artifactId>slf4j-log4j12</artifactId>
-                <version>1.7.22</version>
+                <version>${cs.slf4j.version}</version>
             </dependency>
             <dependency>
                 <groupId>org.springframework</groupId>