chore(scripts): use ssh private key for git sync

[trivikr]

Issue

Internal JS-6078

Description

Uses ssh private key for git sync

Testing

Referred documentation https://github.com/wei/git-sync

Once this commit is merged, we'll verify that git-sync is successful.

On success, we'll delete the following:

• Secret: GIT_SYNC_SOURCE_REPO
• Secret: GIT_SYNC_SOURCE_SSH_PRIVATE_KEY
• Secret: GIT_SYNC_DESTINATION_SSH_PRIVATE_KEY
• Deploy key on this repo: git-sync-source-public-key
• Deploy key on destination repo: git-sync-destination-public-key

The ssh key rotate instructions will be updated to populate private key in secret GIT_SYNC_SSH_PRIVATE_KEY

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[trivikr]

git sync was successful https://github.com/aws/aws-sdk-js-v3/actions/runs/16484986768/job/46607877522
Verified that commit was push on destination repo.

[github-actions]

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