Respect MAX_LEADER_SCHEDULE_STAKES constant

[brooksprumo]

Problem

The MAX_LEADER_SCHEDULE_STAKES constant is not respected. We actually store 6 epochs-worth of stakes, not 5.

(The broader goal/context is to reduce the number of epochs in the epoch stakes cache, which eventually gets serialized into the snapshot.)

Note that MAX_LEADER_SCHEDULE_STAKES = 5 was arbitrarily chosen. Here's its origin: solana-labs#7668 (comment).

Summary of Changes

Actually respect the constant.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.1%. Comparing base (453b75f) to head (6e0d961).
⚠️ Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #8584     +/-   ##
=========================================
- Coverage    83.1%    83.1%   -0.1%     
=========================================
  Files         846      846             
  Lines      368602   368604      +2     
=========================================
- Hits       306674   306671      -3     
- Misses      61928    61933      +5     

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