Fixup failing CI in rust-server due to xmltree dependency

[dsteeley]

The rust-server and rust-server-deprecated depend on serde-xml-rs. This PR updates from a historic fork to the released version. I have replaced the unmerged changes with an appropriate alternative that gives the same function.

to_string_with_namespaces => Use the clear namespace support in the latest serde-xml-rs
wrap_primitives => Uplift the function in forked serde-xml-rs here as this change isn't going to be merged into the upstream crate.

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