initial work for getting a 7.0.0 build to flathub

[Renner0E]

Hey all,

I made some changes to maybe(?) successfully push the 7.0.0 build to flathub.
Right now it fails on validation because flathub changed their metadata guidelines sometime in the past months.

Related PR to update the flatpak manifest and https://buildbot.flathub.org/#/builders/25/builds/14554

developer_name is deprecated but it cannot be removed because some frontends do not support the new developer tag yet see:
https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-developer
https://docs.flathub.org/docs/for-app-authors/metainfo-guidelines/#name-summary-and-developer-name

Verify the metainfo.xml offline

git clone https://github.com/qarmin/czkawka && cd czkawka

appstreamcli validate --explain data/com.github.qarmin.czkawka.metainfo.xml
or
flatpak run --command=flatpak-builder-lint org.flatpak.Builder appstream data/com.github.qarmin.czkawka.metainfo.xml

ERROR before any changes:

I: com.github.qarmin.czkawka:5: summary-has-dot-suffix
Multi functional app to find duplicates, empty folders, similar images, broken files etc.
The component summary should not end with a dot (`.`).

E: com.github.qarmin.czkawka:17: screenshot-image-source-duplicated
There can only be one `source` image per screenshot and language.

E: com.github.qarmin.czkawka:18: screenshot-image-source-duplicated
There can only be one `source` image per screenshot and language.

I: com.github.qarmin.czkawka:25: developer-name-tag-deprecated
The toplevel `developer_name` element is deprecated. Please use the `name` element in a
`developer` block instead.

I: com.github.qarmin.czkawka:~: developer-info-missing
This component contains no `developer` element with information about its author.

✘ Validation failed: errors: 2, infos: 3, pedantic: 1

Maybe also consider changing the app id from com.github.qarmin.czkawka to io.github.qarmin.czkawka. See https://docs.flatpak.org/en/latest/conventions.html