Don't expose metadata for Runtime APIs that haven't been implemented (#6337)

# Description

Prior to this PR, the metadata for runtime APIs was entirely based on
that generated by `decl_runtime_apis`. It therefore didn't take into
account that `impl_runtime_apis` might implement older versions of APIs
than what has been declared.

This PR filters the returned runtime API metadata to only include
methods actually implemented, and also avoids including methods labelled
with `changed_in` (which the previous code was atempting to do already
but not successfully, owing to the attr being removed prior to the
check).

We also change all version related things to be `u32`s (rather than
VERSION being `u32` and `api_version`s being `u64`) for consistency /
ease of comparison.

A test is added which works with both the `enable-staging-api` feature
in api/tests enabled or disabled, to check all of this.

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Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: GitHub Action <[email protected]>