OF-2351: Update log4j to 2.15.0
OF-2351: Update log4j to 2.15.0

This addresses the CVE-2021-44228 "LogJam" vulnerability.

igniterealtime/Openfire,akrherz/Openfire,akrherz/Openfire,igniterealtime/Openfire,igniterealtime/Openfire,guusdk/Openfire,igniterealtime/Openfire,akrherz/Openfire,guusdk/Openfire,guusdk/Openfire,igniterealtime/Openfire,guusdk/Openfire,guusdk/Openfire,akrherz/Openfire,akrherz/Openfire