Improve support for older Safari versions (except speech)

[microbit-robert]

• Update build workflow to use Node 20.x
• Use webkitAudioContext is AudioContext is not defined
• Use audioCtx.createBuffer in place of new AudioBuffer
• Use createBufferSource in place of new AudioBufferSourceNode
• Fix layout that relied on aspect-ratio, closes Avoid use of aspect-ratio for prominent simulator run button #94

An issue with the sample rate still remains for modspeech.

Changes made to accommodate Safari 13 and 14.

[github-actions]

Preview build will be at
https://review-python-simulator.usermbit.org/safari-support/

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[microbit-matt-hillsdon]

This works well except for speech. We've seen it fail on i(Pad)OS Safari <= 14.1 and pass on >= 14.5. We've not been able to test the interval.

Speech uses a 19000 sample rate that's below older Safari's 22050 minimum.

It looks like there might be some options for modspeech we could consider - I'll raise a separate issue to discuss with Damien.