Support int8 migration versions via new int8-versions feature #526

Job	Run time
set-rust-versions	3s
cargo-audit	11s
Build docs	48s
Cargo fmt and clippy	57s
Test postgres (stable)	1m 36s
Test postgres (nightly)	1m 43s
Test mysql-async (stable)	1m 9s
Test postgres (1.75)	1m 41s
Test mysql (stable)	2m 5s
test-macros-and-cli (stable, windows-latest)	3m 43s
Test mysql-async (nightly)	1m 9s
Test mysql (nightly)	2m 19s
test-macros-and-cli (stable, ubuntu-latest)	1m 49s
Test mysql-async (1.75)	1m 10s
Test mysql (1.75)	2m 16s
Test Sqlite (stable)	1m 52s
test-macros-and-cli (nightly, windows-latest)	3m 29s
test-macros-and-cli (nightly, ubuntu-latest)	1m 55s
Test Sqlite (nightly)	1m 52s
Test tiberius (stable)	1m 46s
test-macros-and-cli (1.75, windows-latest)	4m 41s
Test tiberius (nightly)	2m 1s
Test Sqlite (1.75)	1m 44s
Test tokio-postgres (stable)	53s
Test tiberius (1.75)	1m 52s
test-macros-and-cli (1.75, ubuntu-latest)	1m 46s
Test tokio-postgres (nightly)	1m 3s
Test tokio-postgres (1.75)	1m 5s
CI is green	0s
	48m 38s

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