Support int8 migration versions via new int8-versions feature #495

Job	Run time
cargo-audit	18s
set-rust-versions	4s
Build docs	1m 17s
Cargo fmt and clippy	1m 26s
Test tokio-postgres (stable)	58s
Test mysql (stable)	1m 24s
Test tokio-postgres (nightly)	1m 7s
test-macros-and-cli (stable, windows-latest)	1m 59s
Test mysql (nightly)	1m 24s
Test mysql-async (stable)	1m 9s
Test tokio-postgres (1.75)	55s
test-macros-and-cli (stable, ubuntu-latest)	1m 54s
Test mysql (1.75)	1m 29s
Test mysql-async (nightly)	1m 11s
test-macros-and-cli (nightly, windows-latest)	2m 0s
Test mysql-async (1.75)	1m 17s
test-macros-and-cli (nightly, ubuntu-latest)	1m 59s
Test postgres (stable)	34s
Test Sqlite (stable)	1m 11s
test-macros-and-cli (1.75, windows-latest)	1m 57s
Test postgres (nightly)	1m 15s
Test tiberius (stable)	1m 15s
test-macros-and-cli (1.75, ubuntu-latest)	2m 9s
Test postgres (1.75)	1m 16s
Test tiberius (nightly)	1m 20s
Test Sqlite (nightly)	1m 16s
Test tiberius (1.75)	1m 21s
Test Sqlite (1.75)	1m 15s
CI is green	0s
	36m 40s

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