Update default engine to FAISS

[VijayanB]

Description

• Update default engine to FAISS.
• Fixed tests that assumed default engine to be nmslib to either use nmslib explicitly or update test assertion to look for faiss specific artifacts like file extension instead of nmslib artifacts
• removed tests that are just validating default (provided it is nmslib) doesn't support certain features

  Related Issues

  2163

Check List

• [ ] New functionality includes testing.
• [ ] New functionality has been documented.
• [ ] API changes companion pull request created.
• [x] Commits are signed per the DCO using --signoff.
• [x] Public documentation issue/PR created.

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[VijayanB]

Doc issue: https://github.com/opensearch-project/documentation-website/issues/8589

[jmazanec15]

Can you update this too: https://github.com/opensearch-project/k-NN/blob/main/src/main/java/org/opensearch/knn/index/engine/EngineResolver.java#L51

[VijayanB]

Tests are failing not related to this PR. This PR https://github.com/opensearch-project/k-NN/pull/2236 will fix those issue in main. Will rebase once the other PR is merged.

[VijayanB]

Test failure is not due to this change.

[opensearch-trigger-bot[bot]]

The backport to 2.x 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-2.x 2.x
# Navigate to the new working tree
cd .worktrees/backport-2.x
# Create a new branch
git switch --create backport/backport-2221-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 7d3445631591296d00c37ea16351a07ca08ffbd3
# Push it to GitHub
git push --set-upstream origin backport/backport-2221-to-2.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-2221-to-2.x.

[VijayanB]

Failure is due to waiting for this PR to be merged https://github.com/opensearch-project/k-NN/pull/2248