spec: rename the package

Rename the package to `koji-osbuild`, which follows the same scheme
as the OSBS one (`koji-containerbuild`).
The upstream repo has been changed accordingly as well.

koji-osbuild-plugin.spec

@@ -1,6 +1,6 @@
-%global         forgeurl https://github.com/gicmo/koji-osbuild-plugin
+%global         forgeurl https://github.com/gicmo/koji-osbuild
 
-Name:           koji-osbuild-plugin
+Name:           koji-osbuild
 Version:        1
 Release:        1%{?dist}
 Summary:        Koji integration for osbuild composer