Update a few Struts 2.5.x libraries to more recent versions

Hello Apache Struts Team.

This Jira issue is intended to track introduction of newer (compatible) library versions for the Struts 2.5.x line.

Please find below a list of library version updates for the 2.5.x build line, for which the regression build completes successfully.
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Update Struts 2.5.20 build with some newer (compatible) library versions.
Change the main pom.xml library versions for the following:
  - Jackson 2.9.8 -> 2.9.9
  - Log4j2 2.11.1 -> 2.11.2
  - OGNL 3.1.22 -> 3.1.23
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There is an open PR #356 demonstrating the CI build completes with the above library upgrades.

Please review and consider for inclusion in 2.5.21.