Update cairo required version to 1.16 like the targetted Rust crate

Guillaume Gomez requested to merge GuillaumeGomez/librsvg:fix-docsrs-build into main

Fixes #1016 (closed).

It builds locally for me with this change. The cairo-rs feature is using 1.16 but you're requiring the minimum version to be 1.17. I suppose it's what you want but no idea. In any case, if you don't this change, I still have another idea that could work which would be to add an extra [package.metadata.system-deps.docsrs] (or something along the line) which would change the targetted version of cairo.

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