Dependabot: Remove deprecated `reviewers` config #4852

Job	Run time
Ruby analysis (ubuntu-latest, linked)	43s
Ruby analysis (macos-latest, linked)	1m 10s
Ruby analysis (ubuntu-latest, nightly-latest)	44s
Ruby analysis (macos-latest, nightly-latest)	1m 4s
Ruby analysis (macos-latest, default)	1m 12s
Ruby analysis (ubuntu-latest, default)	46s
	5m 39s

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