Keep deprecated feature flag oboe-shared-stdcxx to avoid breaking change
          #977
        
          
      
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#961 removed the feature flag, which is a shame, because that means the next
cpalrelease will be a breaking change. I'd like to get #943 out sooner rather than later (to fix the issues Bevy and others are experiencing withbindgenand the iOS simulator), .So this PR re-adds the feature flag, but with a comment that it is deprecated.
One could argue that changing the backend itself is a breaking change because of the minimum supported version bump. I will argue that it is not, as there is precedent for such bumps in
rustcas well, see e.g. this and this, and disallowing such things in non-breaking versions makes it effectively impossible forcpalto ever reach a stablev1.0.