Rollup merge of #61006 - RalfJung:maybe-uninit, r=Centril
Rollup merge of #61006 - RalfJung:maybe-uninit, r=Centril

adjust deprecation date of mem::uninitialized

In https://github.com/rust-lang/rust/pull/60445 we [decided](https://github.com/rust-lang/rust/pull/60445#issuecomment-488626308) that we'd deprecate for 1.38 instead of 1.40, but I forgot to adjust for that.

graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust