refactor: replaced StringBuffer with StringBuilder.

Author: leekib

aws-serverless-java-container-core/src/main/java/com/amazonaws/serverless/proxy/internal/servlet/AwsCookieProcessor.java

@@ -105,7 +105,7 @@ private Cookie parseCookiePair(String cookiePair) {
 
     @Override
     public String generateHeader(Cookie cookie) {
-        StringBuffer header = new StringBuffer();
+        StringBuilder header = new StringBuilder();
         header.append(cookie.getName()).append('=');
 
         String value = cookie.getValue();