fix(site): Only check for trial plans and without payment mode set (backport #4591)

[mergify]

This fix aims to make sure that switching plans from Paid plan to another paid plan should be easy and only raise error while switching from Trial plan to Paid plan without credits or payment mode added.

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This is an automatic backport of pull request #4591 done by Mergify.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.15%. Comparing base (5bbb974) to head (cdf83c4).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
press/press/doctype/site/site.py	0.00%	2 Missing ⚠️

❌ Your patch status has failed because the patch coverage (0.00%) 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             @@
##           master    #4594       +/-   ##
===========================================
- Coverage   72.85%   51.15%   -21.70%     
===========================================
  Files         101      839      +738     
  Lines       15914    67046    +51132     
  Branches      283      283               
===========================================
+ Hits        11594    34298    +22704     
- Misses       4292    32720    +28428     
  Partials       28       28               

Flag	Coverage Δ
dashboard	72.85% <ø> (ø)

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