Bump Apache Velocity Engine to version 2.x

[ppkarwasz]

Description

We should consider moving away from the unmaintained Velocity 1.x: it starts collecting security advisories, like the one on our Dependabot alerts page.

If we continue to use Maven Site Plugin, we need to bump its version to 4.x (which is in its alpha stage). However the current versions (2.x) of asciidoctor-maven-plugin are not compatible with version 4.x of Maven Site Plugin, so we are blocked until asciidoctor/asciidoctor-maven-plugin#578 is resolved.