Small updates to servlet container test setup. (#12318) (#12447)

Small updates to servlet container test setup. (#12318) (#12447)

* Small updates to servlet container test setup.

- Upgraded to use cargo-maven3-plugin since cargo-maven2-plugin is now
EOL.
- Defined supported packagings as the current jetty-maven-plugin default
only covers war.
- Changed a test pojo to implement Serializable in order to prevent an
error in cargo-maven3-plugin stop-container phase.