Bump struts2-core from 2.5.18 to 2.5.20
Bump struts2-core from 2.5.18 to 2.5.20

Bumps [struts2-core](https://github.com/apache/struts) from 2.5.18 to 2.5.20.
- [Release notes](https://github.com/apache/struts/releases)
- [Commits](https://github.com/apache/struts/commits)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
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