Update migrating to meilisearch cloud from a dump

[CaroFG]

On the "migrating to meilisearch cloud" guide, we explain there are 2 tracks to migrate: the export route and the dump. However, as pointed out by a customer on Discord, the dump option is only available for usage-based plans.

[qdequele]

Interesting it would be nice to bring it back! Do you want to mention the limit so in the documentation?

[CaroFG]

I wanted to know before if it was a product decision to only allow the export route. IMO we should allow both, but not sure what the implications are for a resource-based project