fix #775 - update to java 21

Dockerfile

-FROM eclipse-temurin:17-jre-alpine
+FROM eclipse-temurin:21-jre-alpine
 
 RUN apk update && apk upgrade && \
     rm -rf /var/cache/apk

pom.xml

@@ -46,7 +46,7 @@
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
-        <java.version>17</java.version>
+        <java.version>21</java.version>
 
         <app.versionDate>${maven.build.timestamp}</app.versionDate>
         <maven.build.timestamp.format>dd.MM.yy</maven.build.timestamp.format>