src/merge_source_csvs.py: use YYYY dates for Rayyan

Rayyan calls this the "year" and the ISO8601 format confuses users
with Microsoft Excel anyway. Bah humbug.

src/merge_source_csvs.py

@@ -433,6 +433,9 @@
 logger.info(f"> Normalizing countries...\n")
 df_final["Countries"] = df_final["Countries"].apply(normalize_countries)
 
+# Use YYYY dates for Rayyan
+df_final["Publication date"] = df_final["Publication date"].str.slice(start=0, stop=4)
+
 # Align headers with Rayyan
 df_final = df_final.rename(
     columns={