chore: prepare 2.1 release

[mattinannt]

What does this PR do?

Prepare 2.1 release by adding migration guide, updating package versions and updating docs for self-hosting with the latest changes.

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| Name | Status | Preview | Comments | Updated (UTC) | | :--- | :----- | :------ | :------- | :------ | | **formbricks-cloud** | ⬜️ Ignored ([Inspect](https://vercel.com/formbricks/formbricks-cloud/AoM6p9CdY313auyepqsGZoP7251u)) | [Visit Preview](https://formbricks-cloud-git-feature-21-release-formbricks.vercel.app) | | Jun 11, 2024 2:40pm | | **formbricks-docs** | ⬜️ Ignored ([Inspect](https://vercel.com/formbricks/formbricks-docs/G1Zho39dkHDTU9eo1ggiAL5zcodQ)) | [Visit Preview](https://formbricks-docs-git-feature-21-release-formbricks.vercel.app) | | Jun 11, 2024 2:40pm |

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