qa: fix deprecated log.warn in feature_dbcrash test
qa: fix deprecated log.warn in feature_dbcrash test

Summary:
```
This clears up the following deprecation message when running the test:

test/functional/feature_dbcrash.py:270: DeprecationWarning: The 'warn'
method is deprecated, use 'warning' instead
  self.log.warn("Node %d never crashed during utxo flush!", i)

Git grepping indicates that this was the last remaining use of
`log.warn` in the functional tests.
```

Backport of core PR16491:
https://github.com/bitcoin/bitcoin/pull/16491/files

Test Plan:
  ./test/functional/test_runner.py feature_dbcrash

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4564

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