Updating Skype Client flatpak

• check out https://support.skype.com/en/faq/FA34778/release-notes-for-skype-for-windows-mac-linux-and-web for release notes for the latest version
• get size and SHA256 of target version from https://repo.skype.com/deb/pool/main/s/skypeforlinux/ with curl -v <link.to.deb> | sha256sum
• update com.skype.Client.json's last extra-data section with new URL, size and SHA256
• add a top <release> entry under <releases> to the com.skype.Client.appdata.xml (date attribute value comes from release notes above)
• commit, push & create a PR!