[release-20.0] VTTablet: smartconnpool: notify all expired waiters (#16897)

[vitess-bot]

Description

This is a backport of #16897

[vitess-bot]

Review Checklist

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General

• Ensure that the Pull Request has a descriptive title.
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.68%. Comparing base (f3313b1) to head (c50f707).
Report is 1 commits behind head on release-20.0.

Additional details and impacted files

@@               Coverage Diff                @@
##           release-20.0   #16902      +/-   ##
================================================
+ Coverage         68.66%   68.68%   +0.02%     
================================================
  Files              1541     1541              
  Lines            197964   197965       +1     
================================================
+ Hits             135925   135974      +49     
+ Misses            62039    61991      -48     

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