upstream: Make the Alt-Svc cache configuration required if HTTP/3 is enabled with AutoHttpConfig. (#18153)

upstream: Make the Alt-Svc cache configuration required if HTTP/3 is enabled with AutoHttpConfig.

Risk Level: Low
Testing: Unit tests
Docs Changes: N/A
Release Notes: N/A
Platform Specific Features: N/A

Signed-off-by: Ryan Hamilton <rch@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 19102ffc814204d2d2f98747afa3af3a84a4024c

envoy/extensions/upstreams/http/v3/http_protocol_options.proto

@@ -122,7 +122,9 @@ message HttpProtocolOptions {
     // alternate protocols cache, which is responsible for parsing and caching
     // HTTP Alt-Svc headers. This enables the use of HTTP/3 for origins that
     // advertise supporting it.
-    // TODO(RyanTheOptimist): Make this field required when HTTP/3 is enabled.
+    //
+    // .. note::
+    //   This is required when HTTP/3 is enabled.
     config.core.v3.AlternateProtocolsCacheOptions alternate_protocols_cache_options = 4;
   }