Add back private knn_search API for rest-api-spec generation

[pquentin]

This reverts commit #4276 since _knn_search is still needed in rest-api-spec to run YAML tests against it, as the API is
still available with v8 compatibility mode. However, we make the API private so it does not get back into clients, as it would fail due to v9 compatibility.

Additionally, we fix the compiler to ignore UpdateForV10 in all JSDoc blocks, given this is only meaningful at the ESLint level.

[github-actions]

Following you can find the validation changes against the target branch for the API.

API	Status	Request	Response
knn_search	➕ 🟢	4/4	4/4

You can validate this API yourself by using the make validate target.

[github-actions]

The backport to 9.1 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-9.1 9.1
# Navigate to the new working tree
cd .worktrees/backport-9.1
# Create a new branch
git switch --create backport-5737-to-9.1
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 709ff2494b0fe79f86afc9301529624084ab6297
# Push it to GitHub
git push --set-upstream origin backport-5737-to-9.1
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-9.1

Then, create a pull request where the base branch is 9.1 and the compare/head branch is backport-5737-to-9.1.

[pquentin]

💚 All backports created successfully

Status	Branch	Result
✅	9.1

Questions ?

Please refer to the Backport tool documentation