add dependabot.yml for GitHub Actions update #1927

Job	Run time
conformance	2m 18s
build (2_12, mima)	1m 34s
build (2_12, e2e)	8s
build (2_12, test)	16m 8s
build (2_13, mima)	1m 25s
build (2_13, e2e)	9s
build (2_13, test)	12m 37s
build (3, mima)	6s
build (3, e2e)	5s
build (3, test)	14m 2s
build (2_12, examples_and_formatting)	4m 32s
build (2_13, scala3_compat_test)	11m 32s
build (3, scala3_compat_test)	14m 58s
ci-passed	3s
	1h 19m 37s

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