Spring Replace StringBuffer with StringBuilder in CGLIB fork

This PR replaces StringBuffer with StringBuilder in the CGLIB fork.

I'm aware that any changes on the CGLIB fork are not accepted, but the CGLIB project itself is "unmaintained", so I'm not sure where this sort of change should go.

Comment From: jhoeller

Actually, now that we have a complete fork there just like with ASM, we can apply some general enhancements. We have patched CGLIB significantly in the meantime, and effectively have to maintain it ourselves now. So thanks for the PR, I'll merge it right away!