Adjust mkdocs-insiders version

[why-not-try-calmer]

Can we do without insiders? I always forget why we need to declare it as a build dependency.

[suricactus]

It brings a bit extra things, don't remember exactly what. Last time I checked it was still used.

To test without merging we should rather add the insiders deps in the lint-and-test than commenting it in the build that only runs on master.

[why-not-try-calmer]

This begs for an update of our forks of mkdocs-insiders, zero-ing in on the "recent but not too recent" sweet spot.

[suricactus]

Found the culprit. It is the social plugin.

Tried with and a bit more, without success.:

  - social:
      # See https://squidfunk.github.io/mkdocs-material/setup/setting-up-social-cards/
      concurrency: 1
      debug: true
      log_level: warn
      cards_color:
        fill: "#72ad2e"
        text: "#FFFFFF"

I have reported the issue squidfunk/mkdocs-material#6241 .

[suricactus]

See workaround solution here: #391