fix: Specify minimum rust version

[pistomat]

Unsigned is_multiple_of used in serde_primitives::decode_hex_with_prefix() is stabilized only in 1.87 version of rust as per this issue rust-lang/rust#137383
Building the package with older version fails with

error[E0658]: use of unstable library feature `unsigned_is_multiple_of`
 --> tycho-common/src/serde_primitives.rs:8:24
  |
8 |     if !stripped.len().is_multiple_of(2) {
  |                        ^^^^^^^^^^^^^^
  |
  = note: see issue #128101 <https://github.com/rust-lang/rust/issues/128101> for more information

[propellerci]

This PR is included in version 0.78.4 🎉