[ci] Use prebuilt devcontainer.

feflow often removes older packages breaking our devcontainer build. Current feflow 10 crashes our converter. devcontainer updates will be done manually.

Contains a small bugfix for the web version switcher for the next release.

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behaviour change was documented?
4. API-breaking changes documented here