Add basset:clear to upgrade guide

[jnoordsij]

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When quickly checking out the v7 alpha for possible incompatibilities with my app (first look shows that it's running fine btw, keep up the good work!), I noticed that my local environment erred when trying to render views. Given that this originated from some basset call, I guessed that clearing bassets would solve the issue and it did. I think however that this warrants adding it as a recommended step to the migration guide.

[jcastroa87]

Hello @jnoordsij

Thanks for this PR I will ask @pxpm to check it and if everything is ok we will merge soon.

Cheers.

[pxpm]

Hey @jnoordsij hope you are doing well.

Thanks for the PR 🙏