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.