Upgrade h2 again (#15111)
Closes https://github.com/astral-sh/uv/issues/15056
Following https://github.com/hyperium/h2/pull/858 I'm hoping the
defaults there are more robust and no longer cause the problems reported
in the above issue.
As @konstin noted at
https://github.com/hyperium/h2/issues/856#issuecomment-3160720671, we
may want to tweak the h2 settings during prefetches if we encounter this
problem again.
