[release-21.0] Close zookeeper topo connection on disconnect (#17136)

[vitess-bot]

Description

This is a backport of #17136

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 67.52%. Comparing base (bd8a4b1) to head (3964d90).
Report is 1 commits behind head on release-21.0.

Files with missing lines	Patch %	Lines
go/vt/topo/zk2topo/zk_conn.go	50.00%	2 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release-21.0   #17193      +/-   ##
================================================
+ Coverage         67.50%   67.52%   +0.01%     
================================================
  Files              1570     1570              
  Lines            251298   251294       -4     
================================================
+ Hits             169647   169684      +37     
+ Misses            81651    81610      -41     

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