Replace hardcoded "Sec-WebSocket-Version" with constant

Closes gh-34319

Signed-off-by: Daeho Kwon <trewq231@naver.com>

spring-websocket/src/main/java/org/springframework/web/socket/server/support/AbstractHandshakeHandler.java

@@ -296,7 +296,7 @@ public abstract class AbstractHandshakeHandler implements HandshakeHandler, Life
 
 	protected void handleWebSocketVersionNotSupported(ServerHttpRequest request, ServerHttpResponse response) {
 		if (logger.isErrorEnabled()) {
-			String version = request.getHeaders().getFirst("Sec-WebSocket-Version");
+			String version = request.getHeaders().getFirst(WebSocketHttpHeaders.SEC_WEBSOCKET_VERSION);
 			logger.error(LogFormatUtils.formatValue(
 					"Handshake failed due to unsupported WebSocket version: " + version +
 							". Supported versions: " + Arrays.toString(getSupportedVersions()), -1, true));