.github/workflows/code.yml: switch debian/buster to debian/bookworm #834

Job	Run time
test (2.0)	2m 28s
test-packages (fedora/39)	1m 23s
test (2.5)	2m 37s
test-packages (ubuntu/jammy)	1m 18s
test (2.7)	2m 43s
test-packages (debian/bookworm)	1m 1s
test (3.2)	2m 28s
	13m 58s

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