1.28 Backport H/2 header discard fix

[SeanKilleen]

Commit Message:
Discard the Host header if the :authority header was received to bring Envoy into compliance with https://www.rfc-editor.org/rfc/rfc9113#section-8.3.1

This behavioral change can be reverted by setting runtime flag envoy.reloadable_features.http2_discard_host_header to false.

Risk Level: Low
Testing: Unit Tests
Docs Changes: N/A
Release Notes: Yes
Platform Specific Features: N/A
Runtime guard: envoy.reloadable_features.http2_discard_host_header
Back-ports #30005, supporting #31118

[repokitteh-read-only]

Hi @SeanKilleen, welcome and thank you for your contribution.

We will try to review your Pull Request as quickly as possible.

In the meantime, please take a look at the contribution guidelines if you have not done so already.

🐱

Caused by: #33300 was opened by SeanKilleen.

see: more, trace.

[repokitteh-read-only]

CC @envoyproxy/coverage-shephards: FYI only for changes made to (test/per_file_coverage.sh).
envoyproxy/coverage-shephards assignee is @alyssawilk
CC @envoyproxy/runtime-guard-changes: FYI only for changes made to (source/common/runtime/runtime_features.cc).
CC @envoyproxy/dependency-shepherds: Your approval is needed for changes made to (bazel/.*repos.*\.bzl)|(bazel/dependency_imports\.bzl)|(api/bazel/.*\.bzl)|(.*/requirements\.txt)|(.*\.patch).
envoyproxy/dependency-shepherds assignee is @moderation

🐱

Caused by: #33300 was opened by SeanKilleen.

see: more, trace.

[SeanKilleen]

I apologize for the continued noise. I am going to close this once more because I believe the DCO issue may have been related to be incorrectly opening this under the wrong base branch.