Bump jettyVersion in /wagon/wagon-benchmarks

Bumps `jettyVersion` from 7.4.5.v20110725 to 9.4.21.v20190926.

Updates `jetty-server` from 7.4.5.v20110725 to 9.4.21.v20190926
- [Release notes](https://github.com/eclipse/jetty.project/releases)
- [Commits](https://github.com/eclipse/jetty.project/compare/jetty-7.4.5.v20110725...jetty-9.4.21.v20190926)

Updates `jetty-plus` from 7.4.5.v20110725 to 9.4.21.v20190926
- [Release notes](https://github.com/eclipse/jetty.project/releases)
- [Commits](https://github.com/eclipse/jetty.project/compare/jetty-7.4.5.v20110725...jetty-9.4.21.v20190926)

Signed-off-by: dependabot[bot] <support@github.com>
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