http3: adjust cast for ngtcp2 v0.19.0

ngtcp2 v0.19.0 made size of `ecn` member of `ngtcp2_pkt_info`
an `uint8_t` (was: `uint32_t`). Adjust our local cast accordingly.

Fixes:
```
./curl/lib/vquic/curl_ngtcp2.c:1912:12: warning: implicit conversion loses integer precision: 'uint32_t' (aka 'unsigned int') to 'uint8_t' (aka 'unsigned char') [-Wimplicit-int-conversion]
  pi.ecn = (uint32_t)ecn;
         ~ ^~~~~~~~~~~~~
```

Also bump ngtcp2, nghttp3 and nghttp2 to their latest versions in our
docs and CI.

Ref: 80447281bb
Ref: https://github.com/ngtcp2/ngtcp2/pull/877
Closes #11798

.github/workflows/ngtcp2-linux.yml

@@ -49,9 +49,9 @@ env:
   quictls-version: 3.0.10+quic
   gnutls-version: 3.8.0
   wolfssl-version: master
-  nghttp3-version: v0.14.0
-  ngtcp2-version: v0.18.0
-  nghttp2-version: v1.55.1
+  nghttp3-version: v0.15.0
+  ngtcp2-version: v0.19.0
+  nghttp2-version: v1.56.0
   mod_h2-version: v2.0.21
 
 jobs:

.github/workflows/quiche-linux.yml

@@ -47,9 +47,9 @@ permissions: {}
 env:
   MAKEFLAGS: -j 3
   openssl-version: 3.0.10+quic
-  nghttp3-version: v0.14.0
-  ngtcp2-version: v0.18.0
-  nghttp2-version: v1.55.1
+  nghttp3-version: v0.15.0
+  ngtcp2-version: v0.19.0
+  nghttp2-version: v1.56.0
   quiche-version: 0.17.2
   mod_h2-version: v2.0.21