clippy: Replaces .get(0) with .first()

[brooksprumo]

Problem

There are nightly clippy warnings that will prevent upgrading our stable Rust version.

For this PR, it's this one: https://rust-lang.github.io/rust-clippy/master/index.html#/get(0)

Summary of Changes

Replaces .get(0) with .first().

[codecov]

Codecov Report

Merging #34150 (daa4c9d) into master (6c62ad7) will increase coverage by 0.0%.
Report is 1380 commits behind head on master.
The diff coverage is 90.9%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #34150      +/-   ##
==========================================
  Coverage    81.8%    81.9%              
==========================================
  Files         766      819      +53     
  Lines      209158   220127   +10969     
==========================================
+ Hits       171210   180331    +9121     
- Misses      37948    39796    +1848     

[bw-solana]

LGTM