Make fn `Migrations::max_schema_version` public. #938

Job	Run time
Clippy (main package)	9s
Code coverage	1m 25s
Check	14s
Clippy (all code)	8s
Documentation	14s
Rustfmt	8s
Check with minimal Rust version	44s
Mutation tests (0)	1m 18s
Mutation tests (1)	1m 11s
Test all features	38s
Test feature alpha-async-tokio-rusqlite	37s
Mutation tests (2)	1m 14s
Test examples	47s
Mutation tests (3)	1m 10s
Test feature from-directory	41s
Test no features	39s
	11m 17s

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