StringBuilder shouldn't be marked as IMMUTABLE

[paplorinc]

StringBuilder is definitely NOT immutable, but most of the mutations seem to happen in the parent, in AbstractStringBuilder which don't seem to be considered:

    @Test
    void validate_StringBuilder() {
        var analysisSession = DefaultCachingAnalysisSession.createWithCurrentClassPath(NO_CONFIGURATION);
        assertEquals(NOT_IMMUTABLE, analysisSession.resultFor(Dotted.fromClass(StringBuilder.class)).isImmutable);
    }

results in

expected: <NOT_IMMUTABLE> but was: <IMMUTABLE>
Expected :NOT_IMMUTABLE
Actual   :IMMUTABLE