Check for missing DocBook 5.2 test

[tomschr]

The following change checks for DocBook 5.2 schema.

Without the fix, we will end up with a default validation using DocBook 5.1, even if 5.2 is available. I wouldn't recommend it, we should always validate with the latest version.

In a debugging session with Klaus, I found out that when using daps for the first time without any user config, it has several impacts:

• Any Smart Doc will end up with validation errors. The reason is, all use the new <meta> tag, but that was introduced in 5.2.
• Anybody who wants to validate a Smart Doc needs to create a user config at ~/.config/daps/dapsrc and set the right version.

I hope this change is enough. In the end, the right version (preferably 5.2) should end up in the /etc/daps/config file in the variable DOCBOOK5_RNG_URI. Currently we have here:

DOCBOOK5_RNG_URI="http://docbook.org/xml/5.1/rng/docbookxi.rng"

[tomschr]

I just realized, it was already done in the 4.0-beta2 branch. 🤦‍♂️ Closing it in favor of that branch.