HTTP/2: removed server push (ticket #2432).

Although it has better implementation status than HTTP/3 server push,
it remains of limited use, with adoption numbers seen as negligible.
Per IETF 102 materials, server push was used only in 0.04% of sessions.
It was considered to be "difficult to use effectively" in RFC 9113.
Its use is further limited by badly matching to fetch/cache/connection
models in browsers, see related discussions linked from [1].

Server push was disabled in Chrome 106 [2].

The http2_push, http2_push_preload, and http2_max_concurrent_pushes
directives are made obsolete.  In particular, this essentially reverts
7201:641306096f5b and 7207:3d2b0b02bd3d.

[1] https://jakearchibald.com/2017/h2-push-tougher-than-i-thought/
[2] https://chromestatus.com/feature/6302414934114304

auto/modules

@@ -423,7 +423,6 @@ if [ $HTTP = YES ]; then
 
     if [ $HTTP_V2 = YES ]; then
         have=NGX_HTTP_V2 . auto/have
-        have=NGX_HTTP_HEADERS . auto/have
 
         ngx_module_name=ngx_http_v2_module
         ngx_module_incs=src/http/v2
@@ -444,7 +443,6 @@ if [ $HTTP = YES ]; then
         HTTP_SSL=YES
 
         have=NGX_HTTP_V3 . auto/have
-        have=NGX_HTTP_HEADERS . auto/have
 
         ngx_module_name=ngx_http_v3_module
         ngx_module_incs=src/http/v3