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Fix path-resolution bug in `changenotes.mts validate` by using `getChangenotes` #9062

Job	Run time
Ruby analysis (ubuntu-latest, nightly-latest)	43s
Ruby analysis (macos-latest, nightly-latest)	1m 25s
Ruby analysis (ubuntu-latest, linked)	37s
Ruby analysis (macos-latest, default)	1m 1s
Ruby analysis (ubuntu-latest, default)	40s
Ruby analysis (macos-latest, linked)	1m 10s
	5m 36s