Fix milestone assignment for release branch PRs

[aholstrup1]

Summary

Fix the WorkitemValidation workflow to correctly assign milestones to PRs targeting release branches.

The workflow uses pull_request_target which, with a bare actions/checkout, always checks out the default branch (main). This means PRs targeting release branches (e.g. releases/27.3) get the milestone from main (e.g. "Version 28.0") instead of the correct release branch milestone.

The fix adds ref: ${{ github.event.pull_request.base.ref }} to the checkout steps so the correct branch is checked out.

Work Item(s)

Fixes AB#620875

[github-actions]

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