improvement: fix datetime migration type discovery #27

Job	Run time
ash-ci / mix deps.get	17s
ash-ci / mix hex.audit	24s
ash-ci / MIX_ENV=test mix.compile	2m 11s
ash-ci / MIX_ENV=dev mix.compile	1m 57s
ash-ci / mix format --check-formatted	39s
ash-ci / mix sobelow	39s
ash-ci / mix credo --strict	39s
ash-ci / mix dialyzer	4m 9s
ash-ci / mix spark.cheat_sheets --check	26s
ash-ci / mix spark.formatter --check	27s
ash-ci / mix deps.unlock --check-unused	38s
ash-ci / mix test	1m 46s
ash-ci / Release	0s
	14m 12s

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