add -i: default to the built-in implementation

add -i: default to the built-in implementation

In 9a5315edfdf (Merge branch 'js/patch-mode-in-others-in-c',
2020-02-05), Git acquired a built-in implementation of `git add`'s
interactive mode that could be turned on via the config option
`add.interactive.useBuiltin`.

The first official Git version to support this knob was v2.26.0.

In 2df2d81ddd0 (add -i: use the built-in version when
feature.experimental is set, 2020-09-08), this built-in implementation
was also enabled via `feature.experimental`. The first version with this
change was v2.29.0.

More than a year (and very few bug reports) later, it is time to declare
the built-in implementation mature and to turn it on by default.

We specifically leave the `add.interactive.useBuiltin` configuration in
place, to give users an "escape hatch" in the unexpected case should
they encounter a previously undetected bug in that implementation.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>