Avoid string copies in SpringBootBanner

See gh-12591

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/SpringBootBanner.java

@@ -50,14 +50,14 @@ class SpringBootBanner implements Banner {
 		}
 		String version = SpringBootVersion.getVersion();
 		version = (version == null ? "" : " (v" + version + ")");
-		String padding = "";
+		StringBuilder padding = new StringBuilder();
 		while (padding.length() < STRAP_LINE_SIZE
 				- (version.length() + SPRING_BOOT.length())) {
-			padding += " ";
+			padding.append(" ");
 		}
 
 		printStream.println(AnsiOutput.toString(AnsiColor.GREEN, SPRING_BOOT,
-				AnsiColor.DEFAULT, padding, AnsiStyle.FAINT, version));
+				AnsiColor.DEFAULT, padding.toString(), AnsiStyle.FAINT, version));
 		printStream.println();
 	}