Remove API that was marked obsolete in <= .NET 8

[danmoseley]

It's been since #27529 that we did a global pass to remove API that was marked Obsolete long enough ago. A good definition of long enough ago is "shipped in the last LTS" ie., .NET 8.0, since then even customers that only consume LTS will have had a chance to see the Obsolete attribute.

In a simple grep of release/8.0 vs main, I see roughly 140 that meet this criteria. Probably easiest to do this in chunks by folder.

[danmoseley]

rough grep and diff gave this, with line numbers mostly as of now
https://gist.github.com/danmoseley/e5aed5508af95e8d455cf34f3d90e846

[ilonatommy]

Copilot's PR with current obsoletions: #64059. We can re-run the prompt when we will want to refresh it.