[release-1.6] remove how to enable app http probe instruction

[istio-testing]

Manual cherrypick required.

#7890 failed to apply on top of branch "release-1.6":

Applying: remove how to enable app http probe instruction
Using index info to reconstruct a base tree...
M	content/en/docs/ops/configuration/mesh/app-health-check/index.md
Falling back to patching base and 3-way merge...
Auto-merging content/en/docs/ops/configuration/mesh/app-health-check/index.md
CONFLICT (content): Merge conflict in content/en/docs/ops/configuration/mesh/app-health-check/index.md
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 remove how to enable app http probe instruction
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

[linsun]

probably good to cherry pick together with https://github.com/istio/istio.io/issues/8145

[frankbu]

@linsun, unless it's a critical bug, we don't usually cherrypick back to archived docs because it requires a manual rebuild.

cc @davidhauck

[linsun]

Good point, didn't realize that. Thought it is all automatic @frankbu. @davidhauck please advise.

I can't say it is critical but one of users ran into this and i clarified with him. He is fine now but others could run into it since most of istio users are still on 1.6.x from our lifecycle survey that @therealmitchconnors collected.

[ericvn]

I don't think a rebuild is necessary anymore. We do need to run a script which pulls the changes from the release into master and it builds with the normal build.

[davidhauck]

@ericvn is correct, after this is merged into release-1.6, you will need to follow the steps here: https://github.com/istio/istio.io#updating-an-archive. You should be able to run make build-old-archive-1.6.0 in release-1.7 to accomplish that.

[ericvn]

Note that I have a todo to run this to pick up another change. I can wait to do that process until after this PR would be merged.

[ericvn]

As a test of the command, I did create https://github.com/istio/istio.io/pull/8154 which is an update of the 1.6 repository from the changes that went in after the release branching. I was specifically seeing if changes from https://github.com/istio/istio.io/pull/8055 were there. I did note some other Chinese files were updated, but not sure which commit the are from.

[ericvn]

Are we still worried about updating the 1.6 archived version of the doc for this?