graphics/librsvg2-rust: update to 2.54.1
- Fix oversight in the Minimum Supported Rust Version (MSRV) - this release requires Rust 1.56 or later. This has been true since librsvg 2.53.1, but it was not checked correctly. (Matt Turner)
- #859 - Make rst2man and gi-docgen optional. (Kleis Auke Wolthuizen)
- #856 - Fix documentation comments.