Fix build with latest OpenSearch

[smortex]

OpenSearch recently merged some code that cause buid failure for this
module, as spotted in opensearch-project/anomaly-detection-dashboards-plugin#585.

This change the import to match the new one after
opensearch-project/OpenSearch#9103

[kaituo]

There are still quite a few failures like

D:\a\anomaly-detection\anomaly-detection\src\test\java\org\opensearch\ad\transport\ADStatsTests.java:167: error: cannot find symbol
String json = Strings.toString(builder);
^
symbol: variable Strings
location: class ADStatsTests

[kaituo]

I'll fix the rest of failures in another PR. Merge current PR.

[opensearch-trigger-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-971-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 2839bc2c6f470639b68901086c80cb2152d96558
# Push it to GitHub
git push --set-upstream origin backport/backport-971-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-971-to-2.x.

[smortex]

There are still quite a few failures like

D:\a\anomaly-detection\anomaly-detection\src\test\java\org\opensearch\ad\transport\ADStatsTests.java:167: error: cannot find symbol String json = Strings.toString(builder); ^ symbol: variable Strings location: class ADStatsTests

Dang, I was running ./gradlew run instead of ./gradlew build. It worked so I opened this PR before going to eat.

I think I see your errors now, I am having a look!