Prepare next version: 10.20.0-SNAPSHOT (#4117)

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> [!NOTE]
> **Low Risk**
> Mechanical version-only changes with no runtime or API logic
modifications.
> 
> **Overview**
> Bumps the repo from **10.19.1** to **10.20.0-SNAPSHOT** so `main` is
ready for the next development cycle after a release.
> 
> The version string is updated in `.version`, `gradle.properties`
(`VERSION_NAME`), and `Config.frameworkVersion` (reported SDK version).
CircleCI **docs-deploy** now syncs Dokka output to
`s3://purchases-docs/android/10.20.0-SNAPSHOT`. Example apps (Magic
Weather, Magic Weather Compose, Custom Entitlement Computation) and
`test-apps/sdksizetesting` pin the `purchases` dependency to the new
snapshot in their `libs.versions.toml` files.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
9bcfc4edbe88fe62c402a44219e6e9e7c698921b. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
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