chore: Deprecation of generic-array 0.14 forcing update of digest crate

[nirosys]

Recently a new version of generic-array was released (0.14.8) which introduced a warning to highlight the deprecation of this version of generic-array. generic-array is a transitive dependency that we pull in through the digest crate. digest is used in our ion_hash_tests test, which bubbles up this deprecation warning through our Clippy checks:

Clippy results: 0 ICE, 2 errors, 0 warnings, 0 notes, 0 help
Error: error: use of deprecated struct `digest::generic_array::GenericArray`: please upgrade to generic-array 1.x
  --> tests/ion_hash_tests.rs:50:61
   |
50 |     fn finalize_into(self, out: &mut digest::generic_array::GenericArray<u8, Self::OutputSize>) {
   |                                                             ^^^^^^^^^^^^
   |
   = note: `-D deprecated` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(deprecated)]`


Error: error: use of deprecated struct `digest::generic_array::GenericArray`: please upgrade to generic-array 1.x
  --> tests/ion_hash_tests.rs:58:42
   |
58 |         out: &mut digest::generic_array::GenericArray<u8, Self::OutputSize>,
   |                                          ^^^^^^^^^^^^

We need to update our digest dependency which appears it will have an impact on ion_hash_tests.

As a temporary workaround, I'll be pinning generic-array to version 0.14.7, but we do need to circle back around and address this at some point.