deprecate queryserver-enable-settings-pool flag

[harshit-gangal]

Description

This PR deprecates the queryserver-enable-settings-pool as this is not required to be changed by the user and has defaulted to true from v17 onwards. If disabled could lead to pool exhaustion. The changes are well-tested now and have remained default from v17.

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 5 lines in your changes missing coverage. Please review.

Project coverage is 68.62%. Comparing base (fbbd036) to head (3abd6d4).
Report is 9 commits behind head on main.

Files	Patch %	Lines
go/vt/vttablet/tabletserver/tabletserver.go	61.53%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16280      +/-   ##
==========================================
+ Coverage   68.60%   68.62%   +0.01%     
==========================================
  Files        1544     1544              
  Lines      198016   198008       -8     
==========================================
+ Hits       135849   135875      +26     
+ Misses      62167    62133      -34     

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[deepthi]

I added the release notes label to prevent accidental merge. Once the requested textual change has been made, you may go ahead and remove the label and merge this PR.