Support int8 migration versions via new int8-versions feature #499

Job	Run time
Cargo fmt and clippy	13s
set-rust-versions	4s
Build docs	1m 12s
cargo-audit	19s
Test postgres (stable)	1m 46s
Test postgres (nightly)	1m 47s
Test mysql-async (stable)	1m 8s
Test postgres (1.75)	1m 46s
Test mysql (stable)	2m 6s
test-macros-and-cli (stable, windows-latest)	4m 0s
Test mysql-async (nightly)	1m 12s
Test mysql (nightly)	2m 11s
test-macros-and-cli (stable, ubuntu-latest)	2m 0s
Test mysql-async (1.75)	1m 24s
Test mysql (1.75)	2m 5s
Test Sqlite (stable)	1m 41s
test-macros-and-cli (nightly, windows-latest)	3m 46s
Test Sqlite (nightly)	1m 45s
test-macros-and-cli (nightly, ubuntu-latest)	2m 5s
Test Sqlite (1.75)	1m 43s
test-macros-and-cli (1.75, windows-latest)	4m 5s
Test tokio-postgres (stable)	55s
test-macros-and-cli (1.75, ubuntu-latest)	1m 55s
Test tiberius (stable)	1m 50s
Test tokio-postgres (nightly)	1m 4s
Test tiberius (nightly)	1m 52s
Test tokio-postgres (1.75)	54s
Test tiberius (1.75)	1m 50s
CI is green	0s
	48m 38s

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