bumped minimum version of httpcore

[kylecacciatore]

Summary

Updating version criteria for a minimum version of httpcore to mitigate a downstream vulnerability.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[lovelydinosaur]

Yep, so...

• httpcore v1.0.9 Version 1.0.9 httpcore#1011 unpins the upper bound on h11.
• Ensuring that we don't block h11 v0.16.0 python-hyper/h11@9c9567f

We could issue a new version of httpx that forces httpcore>=1.0.9, tho I don't think that forcing the entire dependency chain to issue new releases all the way through would be necessary/helpful.