* Linux: Remove redundant 'content_rating' tag in AppData file
This fixes a validation error given by appstreamcli:
$ appstreamcli validate io.github.martinrotter.rssguardlite.appdata.xml
E: io.github.martinrotter.rssguardlite:69: tag-duplicated content_rating
https://www.freedesktop.org/software/appstream/docs/chap-Validation.html#asv-tag-duplicated
* Linux: Replace deprecated 'developer_name' tag in AppData file
This is not strictly necessary, but it does fix a deprecation message
shown when validating with appstreamcli, a tool that replaced
appstream-util:
$ appstreamcli validate io.github.martinrotter.rssguardlite.appdata.xml
I: io.github.martinrotter.rssguardlite:8: developer-name-tag-deprecated
https://www.freedesktop.org/software/appstream/docs/chap-Validation.html#asv-developer-name-tag-deprecated
* Linux: Replace appstream-util by appstreamcli to validate AppData file
The appstream-util tool (from appstream-glib) is essentially deprecated
in favor of appstreamcli (from appstream):
https://github.com/hughsie/appstream-glib
* metainfo: add caption to the main screenshot
* metainfo: add link to documentation
* metainfo: add link to release notes
* ci: validate Linux AppStream metadata
