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Fix path-resolution bug in `changenotes.mts validate` by using `getChangenotes` #9042

Job	Run time
Ruby analysis (macos-latest, nightly-latest)	1m 7s
Ruby analysis (ubuntu-latest, linked)	41s
Ruby analysis (macos-latest, linked)	1m 7s
Ruby analysis (ubuntu-latest, nightly-latest)	42s
Ruby analysis (macos-latest, default)	1m 18s
Ruby analysis (ubuntu-latest, default)	29s
	5m 24s