Replace StringBuffer with StringBuilder

[NicolaIsotta]

This also replaces String concatenation inside append methods with chain of append methods.

[wimjongman]

Java already internally converts string concatenation to StringBuilder. The + operator is often more readable.

• https://docs.oracle.com/en/java/javase/21/docs//api/java.base/java/lang/String.html
• https://docs.oracle.com/javase/specs/jls/se17/html/jls-15.html#jls-15.18.1.2
• https://stackoverflow.com/questions/11942368/why-use-stringbuilder-explicitly-if-the-compiler-converts-string-concatenation-t

[NicolaIsotta]

Mixing appends with concatenations is not really readable IMHO. Either one or the other.