Bump MSRV to 1.48

Bump MSRV to 1.48

1.48.0 was released at the end of 2020, nearly 2.5 years ago. It
has been the rustc available on Debian stable since bullseye,
released in 2021. supporting Debian oldstable for more than a year
seems more than sufficient time to give Debian folks to upgrade,
and bullseye is set to become `oldstable` later this year with the
release of `bookworm`, likely this summer.

This also allows us to clean up our MSRV substantially, having a
single MSRV across our crates rather than a number of separate
ones. Sadly, windows already requires 1.49.