Use std-shims to reduce flexible-transcript MSRV to 1.66

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.

crypto/transcript/src/tests.rs

@@ -1,6 +1,8 @@
 use crate::Transcript;
 
 /// Test the sanity of a transcript.
+///
+/// This will panic if sanity checks fail.
 pub fn test_transcript<T: Transcript<Challenge: PartialEq>>() {
   // Ensure distinct names cause distinct challenges
   {