[Backport 2.x] Move decideClient and decideLegacyClient to data source plugin util

[opensearch-trigger-bot]

Backport ddf1a41 from #7079.

[github-actions]

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[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.

Project coverage is 67.59%. Comparing base (353f509) to head (9a7299a).

Files	Patch %	Lines
src/plugins/data/server/index_patterns/routes.ts	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##              2.x    #7138   +/-   ##
=======================================
  Coverage   67.59%   67.59%           
=======================================
  Files        3446     3446           
  Lines       67908    67908           
  Branches    11027    11027           
=======================================
  Hits        45904    45904           
+ Misses      19341    19340    -1     
- Partials     2663     2664    +1     

Flag	Coverage Δ
Linux_1	33.13% <22.22%> (ø)
Linux_2	55.23% <22.22%> (ø)
Linux_3	45.28% <77.77%> (-0.02%)	⬇️
Linux_4	35.02% <22.22%> (ø)
Windows_1	33.15% <22.22%> (ø)
Windows_2	55.18% <22.22%> (ø)
Windows_3	45.30% <77.77%> (+<0.01%)	⬆️
Windows_4	35.02% <22.22%> (ø)

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