Deprecate std.xml

[ghost]

See forum.

[dlang-bot]

Thanks for your pull request and interest in making D better, @berni44! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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[ghost]

I added a release note.

[12345swordy]

@thewilsonator is there any way you can contact @andralex or @atilaneves on this?

[wilzbach]

I'll put my hands in the fire as the leadership doesn't seem to care about it and there's a clear consensus about this in the community.