vtorc: use errgroup in keyspace/shard discovery

[timvaillancourt]

Description

This PR moves to using an errgroup in VTOrc's logic.RefreshAllKeyspacesAndShards() in order for errors to passed up to the caller, also it feels a bit cleaner

Related Issue(s)

#17330

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• 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]

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.

Project coverage is 67.49%. Comparing base (413b953) to head (1511a0a).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtorc/logic/keyspace_shard_discovery.go	0.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17857      +/-   ##
==========================================
+ Coverage   67.44%   67.49%   +0.04%     
==========================================
  Files        1592     1593       +1     
  Lines      258205   258881     +676     
==========================================
+ Hits       174151   174720     +569     
- Misses      84054    84161     +107     

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