[cicd] Migrate to Java21 - docker

Author: dizzzz

exist-docker/src/main/resources-filtered/Dockerfile

@@ -20,7 +20,7 @@
 # Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
 #
 
-FROM gcr.io/distroless/java17-debian12:latest
+FROM gcr.io/distroless/java21-debian12:latest
 
 # Copy eXist-db
 COPY LICENSE /exist/LICENSE

exist-docker/src/main/resources-filtered/Dockerfile-DEBUG

@@ -20,10 +20,10 @@
 # Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
 #
 
-# Use JDK 17 in Debian as our production image gcr.io/distroless/java is based on Debian with just a JRE)
+# Use JDK 21 in Debian as our production image gcr.io/distroless/java is based on Debian with just a JRE)
 FROM debian:bookworm-slim
 RUN apt-get update && apt-get -y dist-upgrade
-RUN apt-get install -y openjdk-17-jdk-headless
+RUN apt-get install -y openjdk-21-jdk-headless
 RUN apt-get install -y expat fontconfig     # Install tools required by FOP
 
 # Copy eXist-db

exist-docker/src/main/resources-filtered/README.md

@@ -17,7 +17,7 @@ The images are based on Google Cloud Platform's ["Distroless" Docker Images](htt
 *   [Docker](https://www.docker.com): `18-stable`
 ### For building
 *   [maven](https://maven.apache.org/): `^3.6.0`
-*   [java](https://www.java.com/): `17`
+*   [java](https://www.java.com/): `21`
 *   [bats](https://github.com/bats-core/bats-core): `^1.1.0` (for testing)
 
 ## How to use