ci(infra): drop redundant opened trigger from issue-link check (#36054)

Remove `opened` from the `require_issue_link.yml` trigger list. New PRs
never have labels at creation time, so the job-level `if` (which
requires the `external` label) always evaluated to skip — producing a
noisy "Skipped" check on every internal PR. Enforcement chains through
`pr_labeler.yml` → `labeled` event, making `opened` a no-op.

## Changes
- Drop `opened` from `pull_request_target.types` in
`require_issue_link.yml` — only `edited`, `reopened`, and `labeled`
remain
- Update header comment to document why `opened` is intentionally
excluded