Bump vega from 5.25.0 to 5.31.0 in /ui-tests (#340)

Bumps [vega](https://github.com/vega/vega) from 5.25.0 to 5.31.0.
- [Release notes](https://github.com/vega/vega/releases)
- [Commits](vega/vega@v5.25.0...v5.31.0)

---
updated-dependencies:
- dependency-name: vega
  dependency-type: indirect
...

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