linux: Fix AppStream metainfo/releases licenses

Previously the comment at the top of the file claimed that these files
are under CC-BY-4.0. But AppStream actually has a field that describes
the license of the metadata, and in this we have:

    <metadata_license>CC0-1.0</metadata_license>

While both licenses are on the AppStream specification's recommended
list of metadata licenses, I think we can err on the side of the more
permissive license here.

https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-metadata_license

Author: wjt

linux/metainfo.xml

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <!--
 SPDX-FileCopyrightText: The Threadbare Authors
-SPDX-License-Identifier: CC-BY-4.0
+SPDX-License-Identifier: CC0-1.0
 -->
 <component type="desktop">
   <name>Threadbare</name>

linux/releases.xml

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <!--
 SPDX-FileCopyrightText: The Threadbare Authors
-SPDX-License-Identifier: CC-BY-4.0
+SPDX-License-Identifier: CC0-1.0
 -->
 <releases>
   <release version="0.1.5" date="2025-10-10">