[chore] upgrade eclipse-temurin from version 20 to 21

[gronke]

Upgrade docbuilder runtime eclipse-temurin from version 20 to 21.

[PeterMosmans]

FYI it makes sense to add automated tests, to ensure stuff still builds after upgrades. Locally I do a testbuild by cloning the pentext repo, and then performing a "report" build, something like:

clone: # Clone PenText to a new directory
	git clone https://github.com/radicallyopensecurity/pentext clone

sample:
	@echo "Creating sample report..." && \
	mkdir -p settings && \
	cp -r sample-report/graphics/* graphics/ && \
	cp -r sample-report/source/* source/

fulltest: clone # Create and build a sample report
	@cp Makefile clone/Makefile && \
	mkdir -p clone/settings && \
	cp settings/fop.conf clone/settings/ && \
	make --silent -C clone clean && \
	make --silent -C clone sample && \
	make --silent -C clone report

I can add a PR for this, but... I personally use a different Makefile (as I'm not using this repo, but I have my own toolchain)