mitigated build warning about `RandomStringUtils.randomAlphanumeric(int)` being deprecated - fixes #102

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The documentation here is not very helpful: https://commons.apache.org/proper/commons-lang/apidocs/org/apache/commons/lang3/RandomStringUtils.html#randomAlphanumeric(int).

Simply adding insecure() as suggested producing a warning about using a static method via an instance. And the fixit for that sends us back to the same code as before. So I used the visible implementation of the deprecated method as basis.