nghttp2: Update to version 1.52.0, fix version header
In !7856 (merged) we forgot to update the nghttp2ver.h
replacements to provide the proper NGHTTP2_VERSION_NUM
. This causes
our build of curl to think it is using a different version of nghttp2
than we provide. Fix the version and add a reminder comment.
While at it, update to 1.52.0.
Fixes: #24419 (closed)
Backport: release