fix: rename doc old_doc bug and validate for rename field

[anandbaburajan]

• Added a new validate arg for rename_field() since I had a use case where I didn't want the validation to be made. For example: in v14 erpnext, there's a doctype with a fieldname "A". In v15, that doctype was moved to a separate lending app after which the fieldname was renamed to "B". Now on v14->v15 migration, on installation of the lending app, I'm running a patch to rename fieldname "A" to "B", but in rename_field(), frappe.db.has_column(doctype, old_fieldname) would fail since the doctype won't have the old fieldname "A".
• If rename_doc() is called with validate=False, an old_doc is not defined error is thrown since old_doc is initialised only if validate=True. So now that one line using old_doc is only executed if validate=True.

[codecov]

Codecov Report

Merging #23248 (ebccab0) into develop (0ff08e4) will decrease coverage by 0.14%.
Report is 24 commits behind head on develop.
The diff coverage is 30.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #23248      +/-   ##
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- Coverage    62.30%   62.16%   -0.14%     
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  Files          770      770              
  Lines        73740    73941     +201     
  Branches      6349     6349              
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+ Hits         45943    45969      +26     
- Misses       24183    24358     +175     
  Partials      3614     3614              

Flag	Coverage Δ
server	70.78% <30.00%> (-0.04%)	⬇️

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