Update README in experimental FIL

[hcho3]

treelite::frontend::LoadXGBoostModel is no longer present in the latest Treelite; use treelite::model_loader::LoadXGBoostModelJSON instead.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Please upload report for BASE (branch-24.10@2f11570). Learn more about missing BASE report.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## branch-24.10 #6052 +/- ## =============================================== Coverage ? 67.06% =============================================== Files ? 195 Lines ? 12920 Branches ? 0 =============================================== Hits ? 8665 Misses ? 4255 Partials ? 0 ```

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[divyegala]

/merge