Backport/2412: Expose the unstable metadata v16

[lexnv]

This PR is a backport for:

• v16: Expose the unstable metadata v16 #5732

cc @paritytech/subxt-team

[github-actions]

This pull request is amending an existing release. Please proceed with extreme caution,
as to not impact downstream teams that rely on the stability of it. Some things to consider:

• Backports are only for 'patch' or 'minor' changes. No 'major' or other breaking change.
• Should be a legit fix for some bug, not adding tons of new features.
• Must either be already audited or not need an audit.

Emergency Bypass

If you really need to bypass this check: add validate: false to each crate
in the Prdoc where a breaking change is introduced. This will release a new major
version of that crate and all its reverse dependencies and basically break the release.