[Refactor] ImmutableOpenMap to j.u.Map in IndexMetadata

[nknize]

Refactors usage of ImmutableOpenMap in IndexMetadata to java.util.Map. This continues the removal of the HPPC dependency in core.

relates #5910

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❕
• TEST FAILURES:

      1 org.opensearch.search.SearchWeightedRoutingIT.testSearchAggregationWithNetworkDisruption_FailOpenEnabled

• URL: https://build.ci.opensearch.org/job/gradle-check/14564/
• CommitID: 15b6a12
  Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[dblock]

Looks pretty straightforward. I looked for downstream references but couldn't find any.

[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 128

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
pushd ../.worktrees/backport-2.x
# Create a new branch
git switch --create backport/backport-7306-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 7b8796aa26cc864bf9731422eca5b166b779a1ef
# Push it to GitHub
git push --set-upstream origin backport/backport-7306-to-2.x
# Go back to the original working tree
popd
# 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-7306-to-2.x.