build: enable GLib deprecation warnings
These were disabled to avoid a large spew of deprecation warnings post
GLib 2.44. That might have been too big a hammer, because it made us
miss us of API from newer GLib than we require.
Let's re-enable the warnings and lower the bottom bound instead.
That way we're get warned about use of API that's too new and also be
warned about things that was deprecated long long ago. We may miss
things that got deprecated in favor of better API after 2.44, but that's
unlikely to be an issue and is definitely better than ignoring
everything altogether.
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