CI: Tell dependabot to update GH Actions #21

Job	Run time
test (ubuntu-latest, 2.4)	36s
test (ubuntu-latest, 2.5)	37s
test (ubuntu-latest, 2.6)	45s
test (ubuntu-latest, 2.7)	41s
test (ubuntu-latest, 3.0)	25s
test (ubuntu-latest, 3.1)	23s
test (ubuntu-latest, 3.2)	22s
test (ubuntu-latest, jruby)	1m 8s
test (ubuntu-latest, truffleruby)	53s
test (windows-latest, 2.4)	4m 14s
test (windows-latest, 2.5)	3m 24s
test (windows-latest, 2.6)	4m 26s
test (windows-latest, 2.7)	2m 53s
test (windows-latest, 3.0)	1m 47s
test (windows-latest, 3.1)	1m 43s
test (windows-latest, 3.2)	3m 19s
test (windows-latest, jruby)	3m 7s
	30m 43s

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