[release-1.15] improve CheckAutoScaleAndReplicaCount

[istio-testing]

Manual cherrypick required.

#40297 failed to apply on top of branch "release-1.15":

Applying: improve CheckAutoScaleAndReplicaCount
Using index info to reconstruct a base tree...
M	operator/pkg/apis/istio/v1alpha1/validation/validation.go
Falling back to patching base and 3-way merge...
No changes -- Patch already applied.
Applying: add release-notes
Using index info to reconstruct a base tree...
Falling back to patching base and 3-way merge...
CONFLICT (add/add): Merge conflict in releasenotes/notes/40297.yaml
Auto-merging releasenotes/notes/40297.yaml
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0002 add release-notes
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".