[dns-server] use the Generation type to represent generations #38780

Job	Run time
check-features	10m 28s
build-docs	7m 24s
check-omicron-deployment (ubuntu-22.04)	8m 57s
check-style	20s
check-omicron-deployment (macos-14)	8m 28s
clippy-lint	9m 29s
	45m 6s

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