Add <releases> tag to client metainfo.xml file

[jwrober]

No related issue. Will make future flatpak updates easier.

[lmoureaux]

Can we generate this automatically so we don't need to remember to update it for every release?
I guess we should set it consistently in all metainfo files, not only the client one.

[lmoureaux]

Oh, and since there's a validation tool for Flatpak, it would be good to run it in the CI (can be a later project though).

[jwrober]

Note this is for stable. For now I think leaving it manual in stable branch is the better approach. I don't want to upset the apple cart so to speak. I want to automate it in all the files w/ CI checks in develop/master branch where we have plenty of time to work out any bugs.

[lmoureaux]

Sorry had missed the target branch!