Instead of accepting a string for just one protocol version accept a list of
supported versions.
* gnu/services/nfs.scm (<nfs-configuration>)[nfs-version]: Remove field and
replace...
[nfs-versions]: ...with this new field.
(nfs-shepherd-services): Pass as many "--nfs-version" options as there are
supported versions.
* doc/guix.texi (Network File System): Document nfs-versions.