Simplify use of Reactor's cast operator

This commit replaces filter(x -> x instanceof C).cast(C.class) with the
built-in ofType(C.class).

See gh-32141

spring-web/src/main/java/org/springframework/http/codec/multipart/PartEventHttpMessageReader.java

@@ -168,8 +168,7 @@ public class PartEventHttpMessageReader extends LoggingCodecSupport implements H
 
 									HttpHeaders headers = headersToken.headers();
 									Flux<MultipartParser.BodyToken> bodyTokens =
-											flux.filter(t -> t instanceof MultipartParser.BodyToken)
-													.cast(MultipartParser.BodyToken.class);
+											flux.ofType(MultipartParser.BodyToken.class);
 									return createEvents(headers, bodyTokens);
 								}
 								else {