Removes unnecessary mut

[brooksprumo]

Problem

There are unnecessary mutable variables that clippy will complain about when upgrading Rust to 1.71.0

warning: variable does not need to be mutable
  --> sdk/program/src/message/compiled_keys.rs:43:17
   |
43 |             let mut meta = key_meta_map.entry(ix.program_id).or_default();
   |                 ----^^^^
   |                 |
   |                 help: remove this `mut`
   |
   = note: `#[warn(unused_mut)]` on by default

Summary of Changes

Remove unnecessary mut

[bw-solana]

LGTM!