Support int8 migration versions via new int8-versions feature #531

Job	Run time
cargo-audit	12s
Cargo fmt and clippy	1m 0s
Build docs	1m 7s
set-rust-versions	7s
Test mysql-async (stable)	1m 28s
Test postgres (stable)	1m 37s
Test mysql (stable)	2m 15s
Test mysql-async (nightly)	1m 15s
Test postgres (nightly)	1m 40s
test-macros-and-cli (stable, windows-latest)	3m 10s
Test mysql (nightly)	2m 23s
Test mysql-async (1.75)	1m 12s
test-macros-and-cli (stable, ubuntu-latest)	1m 53s
Test postgres (1.75)	1m 42s
Test mysql (1.75)	2m 15s
test-macros-and-cli (nightly, windows-latest)	4m 3s
test-macros-and-cli (nightly, ubuntu-latest)	1m 49s
Test Sqlite (stable)	1m 38s
Test tiberius (stable)	1m 56s
test-macros-and-cli (1.75, windows-latest)	3m 37s
Test tokio-postgres (stable)	52s
Test Sqlite (nightly)	2m 7s
Test tiberius (nightly)	1m 50s
test-macros-and-cli (1.75, ubuntu-latest)	2m 4s
Test tokio-postgres (nightly)	1m 2s
Test Sqlite (1.75)	1m 39s
Test tiberius (1.75)	1m 49s
Test tokio-postgres (1.75)	1m 5s
CI is green	1s
	48m 48s

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