KNOX-3189: Update slf4j version to 2.0.13 and log4j2 version to 2.20.0

[bonampak]

KNOX-3189 - Updated slf4j version to 2.0.13 and log4j2 version to 2.20.0.

What changes were proposed in this pull request?

As org.apache.logging.log4j:log4j-slf4j-impl (bridge between SLF4J 1.x and log4j2) needs to be replaced with org.apache.logging.log4j:log4j-slf4j2-impl (bridge between SLF4J 2.x and log4j2), the log4j2 version also needed to be updated (the first available version would have been 2.19.0; for log4j2 2.17.1, the bridge artifact for SLF4J2 does not exist, as slf4j2.x did not exist then).

How was this patch tested?

Inspecting maven dependency tree.
org.slf4j:slf4j-api:jar:2.0.13
org.slf4j:jcl-over-slf4j:jar:2.0.13
org.apache.logging.log4j:log4j-slf4j2-impl:jar:2.20.0

Running manual tests with different log4j configurations using KNOX_GATEWAY_LOG_OPTS in knox-env.sh.
Changing log4j2.debug to true, Configuration status levels, root logger and package-level logs.

Note:

• in our dependency tree, we still have an org.slf4j:slf4j-reload4j:jar:1.7.36:provided from org.apache.hadoop:hadoop-mapreduce-client-core, but that is not a problem as it's not part of our dep release folder.
• ch.qos.reload4j:reload4j:jar:1.2.22 is still there in hadoop-common, but probably we should exclude that as well.

[smolnar82]

LGTM