test: revert Envoy update to verify OOM hypothesis on arm64

[jlojosnegros]

Temporary PR to test whether the Envoy update from PR#399 is causing
the recurring OOM kills in ci/prow/unit-arm on release-1.28.

Since PR#399 merged (envoy-openssl 6b87b3cb -> 8b1cbcbb, version
1.36.10 -> 1.36.11-dev), unit-arm has failed 4 out of 5 attempts
with exit code 137 (SIGKILL/OOM) across #412 and #418.
Before PR#399, unit-arm passed consistently (7/7 PRs).

This PR reverts PR#399 to build against the previous Envoy SHA.
If unit-arm passes, the new Envoy version is the cause.

DO NOT MERGE. Will be closed after collecting the result.

[openshift-ci]

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[jlojosnegros]

/test unit-arm