JitPack build: Use Java 21

[mk868]

I forgot to call sdk use java 21-tem to set the currently used Java version.
After changes in this PR, the build prints the latest Maven and Java 21 to the output - as expected:

Picked up JAVA_TOOL_OPTIONS: -Dfile.encoding=UTF-8 -Dhttps.protocols=TLSv1.2
Apache Maven 3.9.9 (8e8579a9e76f7d015ee5ec7bfcdc97d260186937)
Maven home: /home/jitpack/.sdkman/candidates/maven/current
Java version: 21, vendor: Eclipse Adoptium, runtime: /home/jitpack/.sdkman/candidates/java/21-tem
Default locale: en, platform encoding: UTF-8
OS name: "linux", version: "4.10.0-28-generic", arch: "amd64", family: "unix"

link to output