Replace use of StringBuffer with StringBuilder

With JDK 9, java.util.regex.Matcher received overloaded variants of
appendReplacement() and appendTail() that accept a StringBuilder,
allowing us to replace the use of StringBuffer with StringBuilder.

Closes gh-27822

spring-web/src/main/java/org/springframework/web/util/UriComponents.java

@@ -256,7 +256,7 @@ public abstract class UriComponents implements Serializable {
 			source = sanitizeSource(source);
 		}
 		Matcher matcher = NAMES_PATTERN.matcher(source);
-		StringBuffer sb = new StringBuffer();
+		StringBuilder sb = new StringBuilder();
 		while (matcher.find()) {
 			String match = matcher.group(1);
 			String varName = getVariableName(match);