Changes with nginx 1.21.4 02 Nov 2021
*) Change: support for NPN instead of ALPN to establish HTTP/2 connections has been removed.
*) Change: now nginx rejects SSL connections if ALPN is used by the client, but no supported protocols can be negotiated.
*) Change: the default value of the "sendfile_max_chunk" directive was changed to 2 megabytes.
*) Feature: the "proxy_half_close" directive in the stream module.
*) Feature: the "ssl_alpn" directive in the stream module.
*) Feature: the $ssl_alpn_protocol variable.
*) Feature: support for SSL_sendfile() when using OpenSSL 3.0.
*) Feature: the "mp4_start_key_frame" directive in the ngx_http_mp4_module. Thanks to Tracey Jaquith.
*) Bugfix: in the $content_length variable when using chunked transfer encoding.
*) Bugfix: after receiving a response with incorrect length from a proxied backend nginx might nevertheless cache the connection. Thanks to Awdhesh Mathpal.
*) Bugfix: invalid headers from backends were logged at the "info" level instead of "error"; the bug had appeared in 1.21.1.
*) Bugfix: requests might hang when using HTTP/2 and the "aio_write" directive.