Configure Dependabot for GitHub Actions updates #3

Job	Run time
unit-tests (macos-latest, Release)	28m 53s
benchmarks (skip-run) (macos-latest, Debug)	28s
benchmarks (skip-run) (ubuntu-latest, Debug)	2m 46s
unit-tests (ubuntu-latest, Debug)	19m 23s
benchmarks (skip-run) (macos-latest, Release)	3m 44s
unit-tests (macos-latest, Debug)	2m 11s
unit-tests (ubuntu-latest, Release)	16m 47s
benchmarks (skip-run) (ubuntu-latest, Release)	2m 45s
	1h 16m 57s

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