Fixes `no-std` builds for packages which intended to be `no-std` (without
`alloc`).
Updates a variety of MSRVs to 1.73 due to `flexible-transcript` no longer using
`std-shims` to achieve 1.66 (as `std-shims` requires `alloc`). A future
improvement would be for `std-shims` to have an `alloc` feature and only
provide MSRV shims without it.
flexible-transcript already had a shim to support <1.66. This was irrelevant
since flexible-transcript had a MSRV of 1.73. Due to how clunky it was, it has
been removed despite theoretically enabling an even lower MSRV.
