bootstrap: simplify bootstrap update warning.

It was confusing to sometimes have to `cp -f slingshot/bootstrap`
versus sometimes `mv -f bootstrap.new`.  Perform the copy
automatically, and always advise using mv regardless of source.
* build-aux/bootstrap.slingshot (slingshot_require_bootstrap_uptodate):
Remove stale bootstrap.new, then copy from slingshot/bootstrap
when necessary, in order to simplify update instructions.
* bootstrap: Regenerate.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>