munin: FTBFS with default Java 21

The package munin ftbfs with default Java 21.
The relevant part of the build log:
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cd plugins && /usr/bin/perl Build
Building Munin::Plugins
mkdir -p build/plugins/javalib
touch build-java-stamp
javac -sourcepath plugins/javalib -d build/plugins/javalib -g -source 1.7 -target 1.7 -Xlint plugins/javalib/org/munin/plugin/jmx/GCCountGet.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.
error: Target option 7 is no longer supported. Use 8 or later.
make[2]: *** [Makefile:76: plugins/javalib/org/munin/plugin/jmx/GCCountGet.class] Error 2
make[2]: Leaving directory '/<<PKGBUILDDIR>>'
dh_auto_build: error: make -j1 "INSTALL=install --strip-program=true" CONFIG=debian/Makefile.config "INSTALL_PLUGINS=auto manual snmpauto contrib" returned exit code 2
make[1]: *** [debian/rules:21: override_dh_auto_build] Error 25
make[1]: Leaving directory '/<<PKGBUILDDIR>>'
make: *** [debian/rules:16: binary] Error 2
dpkg-buildpackage: error: debian/rules binary subprocess returned exit status 2
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Build finished at 2023-12-04T10:38:51Z
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