add dependabot.yml (#1595) #1928

Job	Run time
conformance	2m 0s
build (2_12, mima)	1m 20s
build (2_12, e2e)	10s
build (2_12, test)	19m 28s
build (2_13, mima)	1m 30s
build (2_13, e2e)	6s
build (2_13, test)	11m 50s
build (3, mima)	8s
build (3, e2e)	10s
build (3, test)	15m 40s
build (2_12, examples_and_formatting)	3m 29s
build (2_13, scala3_compat_test)	11m 12s
build (3, scala3_compat_test)	8m 36s
ci-passed	2s
	1h 15m 41s

Provide feedback