clippy: rpc lints

[brooksprumo]

Problem

There are new nightly clippy lints in the rpc crate. Refer to #34626 for more information.

warning: writing `&mut Vec` instead of `&mut [_]` involves a new object where a slice will do
   --> rpc/src/optimistically_confirmed_bank_tracker.rs:238:16
    |
238 |         roots: &mut Vec<Slot>,
    |                ^^^^^^^^^^^^^^ help: change this to: `&mut [Slot]`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_arg
    = note: `#[warn(clippy::ptr_arg)]` on by default

warning: `solana-rpc` (lib) generated 1 warning

Summary of Changes

Fix 'em.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (09a94b8) 81.8% compared to head (71fef57) 81.8%.

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #34632     +/-   ##
=========================================
- Coverage    81.8%    81.8%   -0.1%     
=========================================
  Files         824      824             
  Lines      222260   222261      +1     
=========================================
- Hits       181893   181864     -29     
- Misses      40367    40397     +30     

[CriesofCarrots]