Address pandas 2.0 deprecation warnings in FBIHateCrimePublications

[niveditasing]

Summary: Replaced deprecated .fillna(method='ffill') with the direct .ffill() method in the preprocessing scripts for Table 13 and Table 14 & added golden files.

Why this change was made

1. Pandas 2.0+ Compatibility & Deprecation Cleanup
   In modern versions of Pandas (2.0.0 and above), calling .fillna() with the method argument (e.g., method='ffill') is deprecated and
   raises a FutureWarning (or errors in newer releases). The recommended and idiomatic pandas way to forward-fill missing values is calling
   .ffill() directly.

2. Functional Correctness (Propagating Grouped Row Headers)
   The FBI publication Excel files for Tables 13 and 14 use merged/visually-grouped rows where the State and Agency Type are only defined
   on the first row of each section. Subsequent rows for specific cities/agencies are read as NaN by pandas.

 Using .ffill() propagates these parent attributes downwards across the missing rows, ensuring every agency record is correctly resolved

and mapped to its parent State and Agency Type.

[gemini-code-assist]

Code Review

This pull request updates the preprocessing scripts for FBI hate crime tables 13 and 14 by replacing the deprecated pandas fillna(method='ffill') calls with the modern .ffill() method. I have no feedback to provide as the changes are correct and align with modern pandas best practices.