add migrating doc with update gatsby related deps

[chaseadamsio]

calls out what seems to be an issue a number of people are having in #5774. It's not clear that all gatsby-* deps need to be upgraded to next as well.

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[KyleAMathews]

Added some more context for setting plugin's version to next as plugins not hosted here won't probably have a next version.

[jlengstorf]

Hiya @chaseadamsio! 👋

This is definitely late, but on behalf of the entire Gatsby community, I wanted to say thank you for being here.

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