Update required libgpg-error version.

* configure.ac: Set required version of libgpg-error to 1.14.
* README: Update documentation accordingly.
* doc/manual/scute.texi: Likewise.
* doc/website/download.xhtml: Likewise.

Since commit 097a29f, Scute needs the gpgrt_*printf functions,
which were introduced in libgpg-error-1.14.

• Signed-off-by: Damien Goutte-Gattat <dgouttegattat@incenp.org>