Support int8 migration versions via new int8-versions feature #533

Job	Run time
Cargo fmt and clippy	1m 18s
cargo-audit	10s
set-rust-versions	3s
Build docs	1m 15s
Test tokio-postgres (stable)	53s
test-macros-and-cli (stable, windows-latest)	3m 29s
Test tokio-postgres (nightly)	1m 1s
test-macros-and-cli (stable, ubuntu-latest)	1m 40s
Test mysql (stable)	2m 22s
Test tokio-postgres (1.75)	56s
test-macros-and-cli (nightly, windows-latest)	3m 55s
Test mysql-async (stable)	1m 4s
Test mysql (nightly)	2m 13s
Test mysql-async (nightly)	1m 17s
test-macros-and-cli (nightly, ubuntu-latest)	1m 56s
Test mysql (1.75)	2m 14s
Test mysql-async (1.75)	1m 9s
test-macros-and-cli (1.75, windows-latest)	3m 36s
Test postgres (stable)	1m 56s
test-macros-and-cli (1.75, ubuntu-latest)	1m 55s
Test tiberius (stable)	1m 40s
Test Sqlite (stable)	1m 39s
Test postgres (nightly)	1m 43s
Test tiberius (nightly)	1m 53s
Test postgres (1.75)	1m 45s
Test Sqlite (nightly)	1m 49s
Test Sqlite (1.75)	1m 41s
Test tiberius (1.75)	1m 50s
CI is green	0s
	48m 22s

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