Support CC0 licence explicitly in normalized metada for correct default text and version

[cderv]

Another oversight is the the support for CC0 that was added only on the TS part too: #10763

---
format: html
license: "CC0"
---

License: {{< meta license.text >}}

so it won't be CC0 1.0 like the link

Originally posted by @cderv in #13832 (review)

We need to bring support for

licence: CC0

to be normalized to the CC0 type with default version 1.0 so that we get correct default text following #13832