clippy: iter_kv_map

[steviez]

Problem

Work for upgrading to Rust 1.88.0 - see #6850

Summary of Changes

This PR resolves violations of iter_kv_map such as below:
https://rust-lang.github.io/rust-clippy/master/#iter_kv_map

error: iterating on a map's keys
  --> accounts-db/src/ancestors.rs:49:19
   |
49 |         let vec = source.iter().map(|(slot, _)| *slot).collect::<Vec<_>>();
   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `source.keys().map(|slot| *slot)`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#iter_kv_map
   = note: `-D clippy::iter-kv-map` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::iter_kv_map)]`

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.3%. Comparing base (ea9f28e) to head (e8a7ae7).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6855   +/-   ##
=======================================
  Coverage    83.3%    83.3%           
=======================================
  Files         853      853           
  Lines      378267   378267           
=======================================
  Hits       315267   315267           
  Misses      63000    63000           

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