Bump logback-core to 1.5.21

https://logback.qos.ch/news.html#1.5.19
https://logback.qos.ch/news.html#1.5.20
https://logback.qos.ch/news.html#1.5.21

JIRA: LIGHTY-394
Signed-off-by: Peter Suna <peter.suna@pantheon.tech>

Author: PeterSuna

pom.xml

@@ -27,7 +27,7 @@
     <properties>
         <application.main.class>io.lighty.yang.validator.Main</application.main.class>
         <application.attach.zip>true</application.attach.zip>
-        <logback-version>1.5.18</logback-version>
+        <logback-version>1.5.21</logback-version>
     </properties>
 
     <dependencies>