refactor: rename DefaultKeyspace() to CurrentKeyspace()

[systay]

Description

DefaultKeyspace is poorly named - it tells us which database the user has selected with use or using the connection string.

Related Issue(s)

Checklist

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[vitess-bot]

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[harshit-gangal]

should we call it current or selected?

[harshit-gangal]

Taking the name from the error

ERROR 1046 (3D000): No database selected

[codecov]

Codecov Report

Attention: Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.

Project coverage is 67.39%. Comparing base (0726ea6) to head (c61631e).
Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtgate/planbuilder/builder.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17303      +/-   ##
==========================================
- Coverage   67.40%   67.39%   -0.02%     
==========================================
  Files        1574     1574              
  Lines      253221   253277      +56     
==========================================
+ Hits       170690   170700      +10     
- Misses      82531    82577      +46     

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