Bump mkdocs material to v9

[agentbellnorm]

mkdocs-material 9

This PR bumps mkdocs-material, and some of its dependencies, to their latest version.

💥 Impact?

I have suggested to release this as a minor version, reasoning that

• Backstage deployments that do not modify the styling and layout of TechDocs beyond what @backstage/plugin-techdocs does should not have any problems upgrading.
• Although we are now hardcoding some mkdocs-material configuration, I doubt that backstage deployments were depending on them (If you are, please let me know).
• There are currently two known undesired visual changes that users may notice if one were to upgrade mkdocs-techdocs-core without upgrading @backstage/plugin-techdocs, but I don't think they are significant enough to warrant a major release.

Please check out the changelog for more details: https://github.com/backstage/mkdocs-techdocs-core/blob/60f5c4ed6446d7e02e5c0be87228801f87c273be/README.md?plain=1#L148-L157

🚂 PR Sequence

To distribute this change, there are a few PR:s that should be merged in the following order:

1. @backstage/plugin-techdocs: https://github.com/backstage/backstage/pull/17031
2. This PR: https://github.com/backstage/mkdocs-techdocs-core/pull/109
3. spotify/techdocs docker image bump: https://github.com/backstage/techdocs-container/pull/48
4. @backstage/techdocs-node: https://github.com/backstage/backstage/pull/17047

🔧 To consider when upgrading (once all PRs are merged)

To start using mkdocs-techdocs-core:1.2.0, these are the recommended steps

1. Upgrade to version 1.13.0 (or nightly builds after 2023-03-24) of @backstage/plugin-techdocs in your backstage deployment
2. Upgrade your image to spotify/techdocs:1.2
3. Please let us know through gh issues or discord if you encounter any problems

Related issues: https://github.com/backstage/mkdocs-techdocs-core/issues/108, https://github.com/backstage/mkdocs-techdocs-core/issues/63

[axdotl]

Dear @backstage/techdocs-core, is there anything what prevents to get this merged soon?