Fix GetMany when no default index is set

[Mpdreamz]

#4353

Fixed an issue with the GetMany helpers that returned the cartesian product of all ids specified rather
then creating a distinct list if more then one index was targeted.

This PR also updated the routine in the serializer to omit the index name from each item if the index is
already specified on the url in case of multiple indices

This updated routine in the 7.6.0 could throw if you are calling:

client.GetMany<T>(ids, "indexName");

Without configuring ConnectionSettings() with either a default index for T or a global default index.

[github-actions]

The backport to 6.x failed:

The process '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-6.x 6.x
# Navigate to the new working tree
cd .worktrees/backport-6.x
# Create a new branch
git switch --create backport-4463-to-6.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 87c8cdd05ba3ef3818e6cd3444bd71b5de84d443
# Push it to GitHub
git push --set-upstream origin backport-4463-to-6.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-6.x

Then, create a pull request where the base branch is 6.x and the compare/head branch is backport-4463-to-6.x.