Rollup merge of #113740 - tgross35:use-shallow-submodules, r=Mark-Simulacrum

Update `.gitmodules` to use shallow submodule clones

This change makes submodule checkouts shallow by default. This significantly reduces the time needed to do a recursive checkout when `--shallow-submodules` is not specified, such as when `x` is not being used.

Author: matthiaskrgr