[Spark Distribution] Default Assemble artifact to Spark 3

[nknize]

This changes the build.gradle file to default the assembled spark artifacts to use Spark 3 instead of Spark 2. This will enable users to leverage Spark 3 features (e.g., streaming, extensions).

[opensearch-trigger-bot]

The backport to 1.0 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-1.0 1.0
# Navigate to the new working tree
pushd ../.worktrees/backport-1.0
# Create a new branch
git switch --create backport/backport-107-to-1.0
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 81fd694dfa9de0a8a6ee8df0d7c5a3e7b07167c7
# Push it to GitHub
git push --set-upstream origin backport/backport-107-to-1.0
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/backport-1.0

Then, create a pull request where the base branch is 1.0 and the compare/head branch is backport/backport-107-to-1.0.

[opensearch-trigger-bot]

The backport to 2.0 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-2.0 2.0
# Navigate to the new working tree
pushd ../.worktrees/backport-2.0
# Create a new branch
git switch --create backport/backport-107-to-2.0
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 81fd694dfa9de0a8a6ee8df0d7c5a3e7b07167c7
# Push it to GitHub
git push --set-upstream origin backport/backport-107-to-2.0
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/backport-2.0

Then, create a pull request where the base branch is 2.0 and the compare/head branch is backport/backport-107-to-2.0.