Mark HTTP semantic conventions as stable in the YAML definitions

[arminru]

Our semantic convention tooling defines a property called stability for semantic conventions.
Once we finally stabilize our HTTP conventions, the YAML files should be updated to set this property to stable.

See https://github.com/open-telemetry/build-tools/blob/main/semantic-conventions/syntax.md#syntax for details.

[jsuereth]

Is this a blocker for our release?

[arminru]

@jsuereth It's a bit of a chicken and egg thing. Ideally we'd just mark both the MD files and the backing YAML definitions stable in an atomic operation but otherwise either order should be fine as well. If both happens in the same semconv release it will be consistent enough for sure.

Since the upcoming semconv release doesn't mark HTTP stable quite yet it's not a blocker for that release anyway though.