Add tracking issue for `feature(multiple_supertrait_upcastable)`
Move feature(multiple_supertrait_upcastable) to the actual feature gates section (from the internal feature gates section) and give it a tracking issue.
Tracking issue: rust-lang/rust#150833
Fixes https://github.com/rust-lang/rust/issues/150773
This feature is for the `multiple_supertrait_upcastable` lint, which was added as `unstable` without a tracking issue, but was placed in the internal feature gates section. This PR moves its listing to the actual feature gates section and gives it a tracking issue.
If the lint is intended to stay internal-only, then this can be changed to instead mark it as `internal` (and maybe close the tracking issue).
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