MINOR: config: remove experimental status on tune.disable-fast-forward

MINOR: config: remove experimental status on tune.disable-fast-forward

The option was turned to off by default in 2.8 with commit 2f7c82bfd
("BUG/MINOR: haproxy: Fix option to disable the fast-forward"), however
at the same time it should have dropped its experimental status since
the feature is enabled by default. The only goal of the option is to
debug something, like many other tune.xxx options. The option should
still normally not be used without being invited to do so by developers
looking for something specific though.

This could be backported if desired to simplify debugging, though this
has never been needed for now.