Update MakeCode extension location

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The V2 extension is now public. We need to update

https://tech.microbit.org/latest-revision/ and other pages that mention
https://github.com/microbit-foundation/pxt-microbit-v2-extension

to show that to add the extension, you need to search for 'latest micro:bit' or microbit v2 and pick the microbit-v2-extension and link to the readme https://makecode.microbit.org/pkg/microbit-foundation/pxt-microbit-v2-extension

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The MakeCode team aer now porting the extension into MakeCode. Currently at makecode.microbit.org/v3.1, so we should point to this until the features make it into /beta

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Closed by #193