Drop support for GNOME < 43 and simplify glib version handling
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shell-manager: Drop support for XML version parsing
Just assume we have GNOME_DESKTOP_PLATFORM_VERSION (introduced in GNOME 43)
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build: Drop support for older gnome-desktop without the platform version
This was introduced in 43. We keep the check for recent GNOME in the summary() so it's obvious when things are outdated.
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build: Encode glib version in one place only
We keep the check for recent glib in summary() so backports indicate clearly when they don't have it.
This uses the same logic we use in phoc