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

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.

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