bump log4j to 2.15.0 due to CVE-2021-44228

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[mp911de]

Thanks a lot!

[dsteher]

Hi all, thanks for the fix. Do you plan to make a new release for this fix in maven central?

[mp911de]

Yes, once I have time for it.

All logging dependencies are optional so including logstash-gelf into your project will not pull in any log4j/log4j2/… dependencies. The version of log4j2 entirely depends on your project.