Fix hards stop for self-hosted installation

[the-glu]

During our sentry upgrade, I ran into the same issues describled in this post: https://forum.sentry.io/t/migration-errors-9-1-2-to-21-6-1-project-object-has-no-attribute-get-option/14279

This fix the doc by specifying 20.12.1 as an hard stop, solution that also worked in our case :)

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