Add `Ecto.Migration.remove_if_exists/1` (#624) #1660

Job	Run time
unittest (1.15.6, 24.3.4.13)	2m 52s
mysql integration test (1.11.4-erlang-23.3.4.9-alpine-3.16.9, 5.7)	2m 4s
postgres integration test (1.11.4-erlang-23.3.4.9-alpine-3.16.9, 16.2-alpine)	1m 48s
unittest (1.15.6, 26.1.2, lint)	2m 48s
mysql integration test (1.11.4-erlang-23.3.4.9-alpine-3.16.9, 8.0)	2m 4s
mssql integration test (1.11.4-erlang-23.3.4.9-alpine-3.16.9, 2017)	2m 9s
postgres integration test (1.11.4-erlang-23.3.4.9-alpine-3.16.9, 11.11-alpine)	1m 50s
mssql integration test (1.11.4-erlang-23.3.4.9-alpine-3.16.9, 2019)	2m 12s
postgres integration test (1.11.4-erlang-23.3.4.9-alpine-3.16.9, 9.6-alpine)	1m 47s
postgres integration test (1.11.4-erlang-23.3.4.9-alpine-3.16.9, 9.5-alpine)	1m 44s
	21m 18s

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