Auto merge of #51066 - est31:master, r=sfackler

Point to the current box syntax tracking issue

The issue was used for both box syntax as well as placement new.
It got closed due to placement new being unapproved.
So a new one got created for box syntax, yet neither
the unstable book nor feature_gate.rs got updated.
We are doing this now.

r? @aidanhs

diff --cc src/libsyntax/feature_gate.rs
Simple merge