Fix: Deletion timestamp of the Model is updated when a record that has been soft-deleted is deleted again

[iRedds]

Description
Related #5576
If the model provides for soft deletion of records and an attempt is made to re-delete, then timestamps are changed in the record marked as deleted.

This PR adds a condition under which the update will only occur if the deletion timestamp has not yet been set.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[kenjis]

Can you add test case to prove the code works?

Note, we expect all code changes or bug-fixes to be accompanied by one or more tests added to our test suite to prove the code works.
https://github.com/codeigniter4/CodeIgniter4/blob/develop/contributing/pull_request.md#unit-testing

[iRedds]

@kenjis Of all the model tests, I found only life tests.
I don't know if this will be enough.

[kenjis]

@iRedds This is a bug fix. So the workflow is like this:

1. write a test which reproduces the bug, and confirm it fails.
2. fix the bug, and confirm the test passes.

I think your test is enough.