Make r5.xlarge as default instance type

[rishabh6788]

Description

Make r5.xlarge as default instance type.
This default increase it to test benchmark run metrics with our existing performance dashboards that run on r5.2xlarge type instances.
Once we add the option to select instance type in our benchmark run code we will downgrade the defaults here.

Issues Resolved

List any issues this PR will resolve, e.g. Closes [...].

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[opensearch-trigger-bot]

The backport to 1.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-1.x 1.x
# Navigate to the new working tree
pushd ../.worktrees/backport-1.x
# Create a new branch
git switch --create backport/backport-33-to-1.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 fce305519cd84197fcbe2daa60880319aba330ec
# Push it to GitHub
git push --set-upstream origin backport/backport-33-to-1.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/backport-1.x

Then, create a pull request where the base branch is 1.x and the compare/head branch is backport/backport-33-to-1.x.