Update Dependabot configuration for GitHub Actions #5792

Job	Run time
Ruby analysis (macos-latest, linked)	1m 26s
Ruby analysis (ubuntu-latest, nightly-latest)	45s
Ruby analysis (ubuntu-latest, linked)	50s
Ruby analysis (macos-latest, nightly-latest)	1m 16s
Ruby analysis (ubuntu-latest, default)	48s
Ruby analysis (macos-latest, default)	1m 11s
	6m 16s

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