Make sure INSTALL is ASCII plaintext again

This reverts commit 84e93afc ("Switch to UTF-8 for INSTALL") and
reinstates commit c14f2e4a ("Make sure INSTALL is ASCII plaintext")
and regenerates INSTALL.

It turns out that different versions of makeinfo (texinfo/texi2any),
at least versions 7.0.3 and 7.1, put unicode quote glyphs in different
places (specifically whether contractions like you'd, don't, aren't or
you'll use ’ or '). This breaks the make dist target as used for
(snapshot) releases, which have a check on the regenerated INSTALL
file. Using --disable-encoding generates the same plaintext ASCII on
all versions.

An alternative would be to regenerate INSTALL with texinfo 7.1 and
require at least that version. But that seems too soon while various
distros don't have 7.1 yet. We can try again to use UTF-8 for INSTALL
in a couple of years.

Reviewed-by: Florian Weimer <fweimer@redhat.com>