libmatthew-java: FTBFS with OpenJDK 21 due to unsupported javac source/target level 7

libmatthew-java fails to build with OpenJDK 21 because it invokes javac with
the source/target options set to 7. Since OpenJDK 20 the minimum version
supported is 8.

  dpkg-buildpackage
  -----------------

  Command: dpkg-buildpackage --sanitize-env -us -uc -b -rfakeroot
  dpkg-buildpackage: info: source package libmatthew-java
  dpkg-buildpackage: info: source version 0.8.1-2
  dpkg-buildpackage: info: source distribution unstable
  dpkg-buildpackage: info: source changed by Thorsten Alteholz <email address hidden>
   dpkg-source --before-build .
  dpkg-buildpackage: info: host architecture amd64
   fakeroot debian/rules clean
  dh clean --with javahelper
     debian/rules override_dh_auto_clean
  make[1]: Entering directory '/<<PKGBUILDDIR>>'
  dh_auto_clean
          make -j8 clean
  make[2]: Entering directory '/<<PKGBUILDDIR>>'
  rm -rf classes doc
  rm -f .classes .enabledebug .disabledebug *.o *.so *.tar.gz *.jar *.cgi Manifest
  rm -rf libmatthew-java-0.8.1
  make[2]: Leaving directory '/<<PKGBUILDDIR>>'
  #rm -f cx/ath/matthew/debug/Debug.java
  make[1]: Leaving directory '/<<PKGBUILDDIR>>'
     jh_clean
     dh_clean
   debian/rules build
  dh build --with javahelper
     dh_update_autotools_config
     dh_autoreconf
     dh_auto_configure
     jh_linkjars
     debian/rules override_dh_auto_build
  make[1]: Entering directory '/<<PKGBUILDDIR>>'
  /usr/bin/make JAVA_HOME=/usr/lib/jvm/default-java \
                    JAVAC=/usr/lib/jvm/default-java/bin/javac \
                    JAVAH=/usr/lib/jvm/default-java/bin/javah \
                    JAVADOC=/usr/lib/jvm/default-java/bin/javadoc \
                    JAVADOCFLAGS="-quiet -author -link /usr/share/doc/default-jdk-doc/api/" \
                            JVERCFLAGS="-source 1.7" \
                    JAR=/usr/bin/fastjar \
                    PREFIX=/usr \
                    all doc
  make[2]: Entering directory '/<<PKGBUILDDIR>>'
  mkdir -p classes
  /usr/bin/make .disabledebug
  make[3]: Entering directory '/<<PKGBUILDDIR>>'
  mkdir -p classes
  cpp -C -P -DDEBUGSETTING=false < cx/ath/matthew/debug/Debug.jpp > cx/ath/matthew/debug/Debug.java
  /usr/lib/jvm/default-java/bin/javac -source 1.7 -cp classes -d classes cx/ath/matthew/debug/Debug.java cx/ath/matthew/utils/Hexdump.java
  warning: [options] bootstrap class path not set in conjunction with -source 7
  error: Source option 7 is no longer supported. Use 8 or later.
  make[3]: *** [Makefile:110: .disabledebug] Error 2
  make[3]: Leaving directory '/<<PKGBUILDDIR>>'
  make[2]: *** [Makefile:54: .classes] Error 2
  make[2]: Leaving directory '/<<PKGBUILDDIR>>'
  make[1]: *** [debian/rules:7: override_dh_auto_build] Error 2
  make[1]: Leaving directory '/<<PKGBUILDDIR>>'
  make: *** [debian/rules:4: build] Error 2
  dpkg-buildpackage: error: debian/rules build subprocess returned exit status 2
  --------------------------------------------------------------------------------
  Build finished at 2023-09-22T18:02:36Z