Add t-libs-api label to issue filtering

[apiraino]

Discoveredf another label that wasn't filtered out in the list of issues/pr not assigned to a team.

Old version of T-libs filtering labels: -label:T-libs -label:T-libs-impl
After this patch: -label:T-libs -label:T-libs-impl -label:T-libs-api

r? @spastorino

[spastorino]

@apiraino those are part of a re-structure @rust-lang/libs team had. @m-ou-se can you guide @apiraino about the right labels to use?

[m-ou-se]

• T-libs was renamed to T-libs-api
• T-libs-impl was renamed to T-libs

So, before we had a generic libs label and a specific label for implementation issues. Now we have a generic libs label and a specific one for the api issues.

[m-ou-se]

Or maybe this is clearer:

let t_libs_api = std::mem::replace(&mut t_libs, t_libs_impl);

[apiraino]

r? @spastorino I think I have addressed the comment