nghttp2: Update import script to fix version header

In commit 66c9536775 (nghttp2: Update script to get nghttp2 1.50.0,
2022-10-31, v3.26.0-rc1~455^2~2) 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.

Issue: #24419