fix (ci): upgrade probot version in CI

[deependujha]

What does this PR do?

Fixes #<issue_number>

Problem

Probot fails to post comments on PRs opened from forks (example failing run). This is because the pull_request trigger runs with a read-only token for fork PRs, which doesn't have permission to write comments.

PRs from branches within the same repo are unaffected (example working comment).

Fix

Switch to pull_request_target, which runs in the context of the base repo and gets a read-write token. Probot only polls check statuses and posts comments; it never checks out PR code, so this is safe to do.

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📚 Documentation preview 📚: https://pytorch-lightning--21670.org.readthedocs.build/en/21670/

[Copilot]

Pull request overview

This PR updates the Probot workflow trigger so Probot can post comments on pull requests opened from forks by running in the base repository context.

Changes:

• Switch the workflow trigger from pull_request to pull_request_target.
• Add an inline note explaining why the trigger change is considered safe for this workflow.

Comments suppressed due to low confidence (1)

.github/workflows/probot-check-group.yml:9

• With pull_request_target, github.ref is the base branch ref and github.head_ref is only the source branch name (not fork-qualified), so the current concurrency group can collide across different fork PRs that use the same branch name, causing unrelated runs to cancel each other. Consider using a unique key like the PR number (github.event.pull_request.number) in the concurrency group.

  pull_request_target:
    types: [opened, reopened, ready_for_review, synchronize] # added `ready_for_review` since draft is skipped

concurrency:
  group: ${{ github.workflow }}-${{ github.ref }}-${{ github.head_ref }}

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[deependujha]

update: upgrading to probot v5.5 fixed the issue, without requiring pull_request_target, hence dropped it. Still unable to comment on fork prs, but it isn't very necessary either.