`html_meta` and `substitutions` topmatter keys deprecated, moved under `myst` key

[stevepiercy]

Plone Documentation versions affected

• [ ] 5.x
• [X] 6.x

Description

The release of myst-parser 0.18.0 on 2022-06-07 introduced a breaking change. From https://myst-parser.readthedocs.io/en/latest/develop/_changelog.html#breaking-changes

The top-level html_meta and substitutions top-matter keys have also been deprecated (i.e. they will still work but will emit a warning), as they now form part of the myst config, e.g.

html_meta:
  "description lang=en": "metadata description"
substitutions:
  key1: I'm a **substitution**

is replaced by:

myst:
  html_meta:
    "description lang=en": "metadata description"
  substitutions:
    key1: I'm a **substitution**

This will need to be pushed out to all other repos that are imported by this one, as well as training.

• [ ] documentation
• [ ] plone.api
• [ ] plone.restapi
• [ ] volto
• [ ] training

Related issues:

• https://github.com/plone/documentation/issues/1147
• https://github.com/plone/documentation/issues/1243