Specify utf-8 in script tag

[not522]

In our project (Optuna), plotly shows garbled text because of no specification for the encoding. This PR set charset to utf-8 in the script tag.

related issues:

• Issue while displaying negative numbers , minus symbol getting displayed as strange character plotly.js#5327
• Weird marker rendering glitches in scatter3d plotly.js#6491

Code PR

• I have read through the contributing notes and understand the structure of the package. In particular, if my PR modifies code of plotly.graph_objects, my modifications concern the codegen files and not generated files.
• I have added tests (if submitting a new feature or correcting a bug) or
  modified existing tests.
• For a new feature, I have added documentation examples in an existing or
  new tutorial notebook (please see the doc checklist as well).
• I have added a CHANGELOG entry if fixing/changing/adding anything substantial.
• For a new feature or a change in behaviour, I have updated the relevant docstrings in the code to describe the feature or behaviour (please see the doc checklist as well).

[archmoj]

Thanks for the PR.
💃