Rollup merge of #139547 - joshtriplett:library-tracking-issue-template, r=Amanieu

Update library tracking issue template to set S-tracking-unimplemented

This will help people notice the `S-tracking-*` labels, and if the
tracking issue *is* implemented, they can change the label.

Discussed in a `@rust-lang/libs-api` meeting.

r? `@Amanieu`

Author: matthiaskrgr