Should fix 1034.

## Investigation

It seems PR previews _are_ being cleaned up correctly for _merged_ PRs;
the teardown action is running, and the previews are no longer
accessible after merging (see following screenshot as example).

<img width="300" alt="Screenshot 2024-06-07 at 13 35 48"
src="https://github.com/Qiskit/documentation/assets/36071638/0066b309-9432-4213-81bd-1836521a8900">

It seems the problem is if a PR is closed but not merged (e.g. #1468,
#1380, and #1289 still have active previews). I believe this is the
reason:
https://github.com/orgs/community/discussions/26657#discussioncomment-3252753.


## Solution

This PR tries to fix this by using the `pull_request_target` event to
trigger teardowns. This should trigger correctly when PRs are closed.

We need to be careful with this event as it allows untrusted PRs to
trigger events using secrets. Since this action runs on the target
branch (rather than a merge commit of the target and PR branches), an
untrusted user shouldn't be able to do anything malicious, but we need
to make sure we **never** checkout the PR branch with this event type,
or read any user-defined inputs (such as the PR title) as it could lead
to an injection. I think this is unlikely, but I've left a warning in
the action just in case.

See frankharkins#7 for a
demonstration. The "Preview teardown" step ran after the PR was closed.

Should fix 1034.

## Investigation

It seems PR previews _are_ being cleaned up correctly for _merged_ PRs;
the teardown action is running, and the previews are no longer
accessible after merging (see following screenshot as example).

<img width="300" alt="Screenshot 2024-06-07 at 13 35 48"
src="https://github.com/Qiskit/documentation/assets/36071638/0066b309-9432-4213-81bd-1836521a8900">

It seems the problem is if a PR is closed but not merged (e.g. Qiskit#1468,
Qiskit#1380, and Qiskit#1289 still have active previews). I believe this is the
reason:
https://github.com/orgs/community/discussions/26657#discussioncomment-3252753.


## Solution

This PR tries to fix this by using the `pull_request_target` event to
trigger teardowns. This should trigger correctly when PRs are closed.

We need to be careful with this event as it allows untrusted PRs to
trigger events using secrets. Since this action runs on the target
branch (rather than a merge commit of the target and PR branches), an
untrusted user shouldn't be able to do anything malicious, but we need
to make sure we **never** checkout the PR branch with this event type,
or read any user-defined inputs (such as the PR title) as it could lead
to an injection. I think this is unlikely, but I've left a warning in
the action just in case.

See #7 for a
demonstration. The "Preview teardown" step ran after the PR was closed.