[BUG] Backport CI is duplicating workflow runs

[dbwiddis]

What is the bug?

Backport workflows are running on both push and pull_request

How can one reproduce the bug?

1. Push to the backport branch (which happens on a merged PR on main).
2. Observe duplicated runs for CI and Validate Gradle Wrapper
   https://github.com/opensearch-project/opensearch-ai-flow-framework/pull/98/checks

What is the expected behavior?

Each workflow only runs once

What is your host/environment?

GitHub Actions

Do you have any screenshots?

Do you have any additional context?

The backport PR branch should be excluded from push in branches-ignore similar to how the remediate branch is excluded in both the CI and validate gradle wrapper workflows.