fix(signup): To avoid lock wait timeout error during signup

[Bowrna]

Getting preferred site can return same site for concurrent signups causing issue. Added for_update and skip_locked to ensure we don't pick same site and reduced the limit to less number too

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.50%. Comparing base (3e0501b) to head (b6b318e).
⚠️ Report is 34 commits behind head on develop.

Files with missing lines	Patch %	Lines
press/press/doctype/site/site.py	50.00%	1 Missing ⚠️
press/saas/doctype/product_trial/product_trial.py	0.00%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (33.33%) is below the target coverage (75.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5280      +/-   ##
===========================================
- Coverage    50.51%   50.50%   -0.01%     
===========================================
  Files          847      847              
  Lines        69050    69049       -1     
  Branches       275      275              
===========================================
- Hits         34880    34876       -4     
- Misses       34149    34152       +3     
  Partials        21       21              

Flag	Coverage Δ
dashboard	72.67% <ø> (ø)

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