utils: Use Guile-zlib for gzip compression and decompression.

Fixes <https://bugs.gnu.org/46967>.

* guix/utils.scm (decompressed-port): Call 'make-zlib-input-port'
instead of invoking %GZIP.
(compressed-output-port): Call 'make-zlib-output-port' instead of
invoking %GZIP.
* doc/guix.texi (Requirements): Require Guile-zlib >= 0.1.0.