Auto merge of #50522 - alexcrichton:beta-next, r=alexcrichton
[beta] Prepare the 1.27.0 beta release
This commit prepares the 1.27.0 beta release by doing:
* Update the release channel to `beta`
* Update Cargo's submodule
* Update `stdsimd`'s submodule
* Update the bootstrap compiler to the freshly minted 1.26.0 stable release