fixes LoanBrokerSet debtMaximum limits

[Tapanito]

Adds a check to ensure that when setting debtMaximum, it does not go below debtTotal.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.1%. Comparing base (5224636) to head (1208b57).

Additional details and impacted files

@@                      Coverage Diff                       @@
##           ximinez/lending-XLS-66-ongoing   #6116   +/-   ##
==============================================================
  Coverage                            79.1%   79.1%           
==============================================================
  Files                                 839     839           
  Lines                               71370   71376    +6     
  Branches                             8322    8314    -8     
==============================================================
+ Hits                                56453   56471   +18     
+ Misses                              14917   14905   -12     

Files with missing lines	Coverage Δ
src/xrpld/app/tx/detail/LoanBrokerSet.cpp	99.1% <100.0%> (+0.1%)	⬆️

... and 3 files with indirect coverage changes

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