sysutils/conky: update to 1.21.7

ChangeLog: https://github.com/brndnmtthws/conky/releases/tag/v1.21.7

Bug fixes

 * Fix logic error in nvidia frequency calculation
 * Fix logic bug in cache_nvidia_string_value_update()
 * Stop trying to access filesystems that are no longer in dynamically generated
   conky text