Bump d3-color and @ssthouse/vue-tree-chart in /webapp/frontend

Bumps [d3-color](https://github.com/d3/d3-color) to 3.1.0 and updates ancestor dependency @ssthouse/vue-tree-chart. These dependencies need to be updated together.


Updates `d3-color` from 1.0.3 to 3.1.0
- [Release notes](https://github.com/d3/d3-color/releases)
- [Commits](d3/d3-color@v1.0.3...v3.1.0)

Updates `@ssthouse/vue-tree-chart` from 0.5.4 to 0.6.8

---
updated-dependencies:
- dependency-name: d3-color
  dependency-type: indirect
- dependency-name: "@ssthouse/vue-tree-chart"
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>