Update vega

[clarasb]

This PR updates vega to v6 and its corresponding dependencies.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.17%. Comparing base (63037e1) to head (e4426f8).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #131      +/-   ##
==========================================
- Coverage   92.71%   92.17%   -0.54%     
==========================================
  Files          86       86              
  Lines        1496     1496              
  Branches      225      225              
==========================================
- Hits         1387     1379       -8     
- Misses         91       98       +7     
- Partials       18       19       +1     

Flag	Coverage Δ
backend	100.00% <100.00%> (ø)
frontend	83.75% <ø> (-1.12%)	⬇️

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[b-yogesh]

Please look/respond to my comment. Does it works fine with xcube-viewer too? There is one test for VegaChart that is missing in the Github CI, is that intentional?