Update Envoy SHA to HEAD of istio/envoy:release-1.3

[rlenglet]

Cherry-pick the Envoy SHA update from the release-1.3.6-patch branch / 1.3.6 tag.

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[rlenglet]

The ci/circleci: macos test fails due to the Python version. This is not a new failure, it's not due to this PR.