Deprecated mathvariant is still used in Katex #4043

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Describe the bug:
Mathvariant is deprecated in chrome and set to be deprecated in firefox and as a result whenever katex decides to use this, chrome doesn't render properly and firefox throws deprecation error in console.

To Reproduce:
Steps to reproduce:

1. Try rendering \mathbb{R} in demo in mathml setting
2. Notice that it renders plain R wrongly in chrome
3. Notice that it renders in firefox but throws deprecation notice for mathvariant in console

Expected behavior:
To not use mathvariant, pandoc for example somehow doesn't give same error, possibly because it doesn't use katex internally to convert equations to mathml.

Environment (please complete the following information):

• KaTeX Version: 0.16.21
• Device: Desktop
• OS: Windows 11
• Browser: Chrome, Firefox
• Version: Chrome 134.0.6998.178, Firefox 137.0

Additional context:
Katex installed using npm and called using node. Behavior exactly reproducible in demo, use \mathbb{R} and render as mathml.