fix: remove readonly from billing address

[18alantom]

Customers may have multiple billable addresses from which the user would like to select one. Hence removed the read-only from this field.

[codecov]

Codecov Report

Merging #27872 (41fefa3) into develop (35e30bd) will increase coverage by 4.92%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop   #27872      +/-   ##
===========================================
+ Coverage    49.60%   54.52%   +4.92%     
===========================================
  Files         1248     1248              
  Lines        67519    67519              
===========================================
+ Hits         33494    36817    +3323     
+ Misses       34025    30702    -3323     

Impacted Files	Coverage Δ
...tch_item_expiry_status/batch_item_expiry_status.py	69.81% <0.00%> (-24.53%)	⬇️
...wise_balance_history/batch_wise_balance_history.py	69.49% <0.00%> (-23.73%)	⬇️
erpnext/stock/report/stock_ageing/stock_ageing.py	87.76% <0.00%> (-5.76%)	⬇️
...eport/item_variant_details/item_variant_details.py	84.52% <0.00%> (-3.58%)	⬇️
...value/warehouse_wise_item_balance_age_and_value.py	92.59% <0.00%> (-2.47%)	⬇️
erpnext/hr/doctype/staffing_plan/staffing_plan.py	91.13% <0.00%> (-2.20%)	⬇️
.../hr/doctype/upload_attendance/upload_attendance.py	45.11% <0.00%> (-1.51%)	⬇️
...rpnext/stock/report/stock_balance/stock_balance.py	78.82% <0.00%> (-0.59%)	⬇️
erpnext/support/doctype/issue/issue.py	63.59% <0.00%> (-0.49%)	⬇️
erpnext/stock/doctype/serial_no/serial_no.py	78.92% <0.00%> (-0.41%)	⬇️
... and 149 more