iceberg: use uppercase strings to represent data file format

Iceberg is a bit inconsistent when defining this. Some libraries
(pyiceber in particular) requires it to be upper case string. For
Redpanda the case doesn't matter as we always lower case the value when
parsing it.

Signed-off-by: Michał Maślanka <michal@redpanda.com>

src/v/iceberg/manifest_entry_values.cc

@@ -143,11 +143,11 @@ data_file_content_type content_from_int(int c) {
 iobuf format_to_str(data_file_format f) {
     switch (f) {
     case data_file_format::avro:
-        return iobuf::from("avro");
+        return iobuf::from("AVRO");
     case data_file_format::orc:
-        return iobuf::from("orc");
+        return iobuf::from("ORC");
     case data_file_format::parquet:
-        return iobuf::from("parquet");
+        return iobuf::from("PARQUET");
     }
 }