Core Change: ERPNext Account can only be linked to one Bank Account

[marination]

Currently we allow many to one linking between Bank Accounts and an ERPNext Account
Solution:
On Linking account, prompt user a Bank Account selection as well.

Will work for existing users until they re-link accounts. Add a Note on migrate

[barredterra]

Here's a related Error Log:

Title

Kosma Error - Bank Account Update

Error

Traceback (most recent call last):
  File "apps/banking/banking/klarna_kosma_integration/utils.py", line 209, in update_account
    account.save()
  File "apps/frappe/frappe/model/document.py", line 305, in save
    return self._save(*args, **kwargs)
  File "apps/frappe/frappe/model/document.py", line 340, in _save
    self.run_before_save_methods()
  File "apps/frappe/frappe/model/document.py", line 1045, in run_before_save_methods
    self.run_method("validate")
  File "apps/frappe/frappe/model/document.py", line 915, in run_method
    out = Document.hook(fn)(self, *args, **kwargs)
  File "apps/frappe/frappe/model/document.py", line 1267, in composer
    return composed(self, method, *args, **kwargs)
  File "apps/frappe/frappe/model/document.py", line 1249, in runner
    add_to_return_value(self, fn(self, *args, **kwargs))
  File "apps/frappe/frappe/model/document.py", line 912, in fn
    return method_object(*args, **kwargs)
  File "apps/erpnext/erpnext/accounts/doctype/bank_account/bank_account.py", line 29, in validate
    self.validate_account()
  File "apps/erpnext/erpnext/accounts/doctype/bank_account/bank_account.py", line 36, in validate_account
    frappe.throw(
  File "apps/frappe/frappe/__init__.py", line 538, in throw
    msgprint(
  File "apps/frappe/frappe/__init__.py", line 506, in msgprint
    _raise_exception()
  File "apps/frappe/frappe/__init__.py", line 452, in _raise_exception
    raise raise_exception(msg)
frappe.exceptions.ValidationError: '<strong>1800 - Bank - TC</strong>' account is already used by <strong><a href="/app/bank-account/REDACTED_IBAN">REDACTED_IBAN</a></strong>. Use another account.

This one can be resolved by manually removing the used Account from other existing Bank Accounts.