Update atom_syndication to 0.12

[parksb]

Currently, cargo check command prints following warning:

warning: the following packages contain code that will be rejected by a future version of Rust: quick-xml v0.12.4, quick-xml v0.20.0
note: to see what the problems were, use the option `--future-incompat-report`, or run `cargo report future-incompatibilities --id 1`

This PR updates atom_syndication to 0.12 version that depends on quick_xml v0.30.