Keep track of the following unmaintained indirect dependencies

[ozgunozerk]

These are for the rust backend, and are NOT security issues, but rather warnings about unmaintained dependencies. It is good to have an issue for them to keep track of the progress. We can upgrade them when the appropriate time comes.

• ansi_term (opened an issue on substrate: RUSTSEC-2021-0139: ansi_term is Unmaintained paritytech/polkadot-sdk#31)
• xml-rs (Using quick-xml ebarnard/rust-plist#68, and also: Maintenance of xml-rs netvl/xml-rs#221)