chore: enable dependabot for mix dependencies. #19

Job	Run time
ash-ci / mix deps.get	24s
ash-ci / mix hex.audit	21s
ash-ci / MIX_ENV=test mix.compile	1m 12s
ash-ci / MIX_ENV=dev mix.compile	1m 28s
ash-ci / mix dialyzer	3m 51s
ash-ci / mix credo --strict	37s
ash-ci / mix format --check-formatted	38s
ash-ci / mix sobelow	25s
ash-ci / mix spark.cheat_sheets --check	27s
ash-ci / mix spark.formatter --check	38s
ash-ci / mix test	1m 9s
ash-ci / mix deps.unlock --check-unused	27s
ash-ci / mix docs	31s
ash-ci / Deploy docs to GitHub pages.	0s
ash-ci / Release	0s
	12m 8s

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