Update the extension versions in flatpak build instructions to 21.08
The extensions need to match the SDK version, that was updated to org.gnome.Sdk//41 in e5e02b13. (Version 41 is based on org.freedesktop.Sdk//21.08)
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The extensions need to match the SDK version, that was updated to org.gnome.Sdk//41 in e5e02b13. (Version 41 is based on org.freedesktop.Sdk//21.08)