Simplify StringBuilder and StringBuffer approximations

[Damtev]

The current approximations for java.lang.StringBuilder and java.lang.StringBuffer use an instance of java.lang.String as an internal storage and the java.lang.String#concat method for approximating java.lang.AbstractStringBuilder#append methods. Such approximation is still very complex for symbolic analysis, taking into account that the java.lang.String#concat uses the non-trivial method java.lang.StringConcatHelper#simpleConcat (that includes even jdk.internal.misc.Unsafe invocations`).

Instead of current approximations, it is suggested to use a hand-written dynamic array of characters for simpler approximations. For reference, use org.utbot.engine.overrides.strings.UtStringBuilder implementation in UTBotJava.