Unstable `gen_future` Feature Tracking
This PR removes the reference to the closed tracking issue **#50547** for the `gen_future` feature. Since `gen_future` is an internal feature used in async block desugaring, it does not require a public tracking issue.
#### Changes:
- Replaced `issue = "50547"` with `issue = "none"` in **library/core/src/future/mod.rs**.
- Ensures that it is correctly identified as an internal feature.
#### Rationale:
With this change, the Unstable Book will now state:
> *"This feature has no tracking issue and is therefore likely internal to the compiler, not being intended for general use."*
Closes **#76249**. 🚀🦀
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