Bump PLOTLYJS_VERSION

[Stefan Pofahl]

Describe the bug
PlotlyBase.jl currently refers to v2.3.0 instead of Plotly.js v2.4.1

Is there a chance that there will be next version of both PlotlyJS as well as PlotlyBase.jl?

See also thread:
legend-groupclick-toggleitem-doesnt-seem-to-work-in-plotlyjs