Fix error code received from vttablets on context expiration

[GuptaManan100]

Description

This PR fixes the error codes returned when context expires on vttablet even before the execution of the query starts. Previously we would return the error by calling fmt.Errorf, which on being translated to grpc error codes, would result in Code_UNKNOWN. Instead if we use vterrors.Errorf and use vtrpcpc.Code_CANCELLED, we can get a better error code.

Related Issue(s)

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[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 68.93%. Comparing base (e89f684) to head (3b342df).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vttablet/tabletserver/connpool/dbconn.go	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16685      +/-   ##
==========================================
- Coverage   68.93%   68.93%   -0.01%     
==========================================
  Files        1564     1565       +1     
  Lines      201371   201533     +162     
==========================================
+ Hits       138821   138922     +101     
- Misses      62550    62611      +61     

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[deepthi]

LGTM, but other reviews should be addressed.