Prepare next version: 10.20.0-SNAPSHOT (#4105)

<!-- CURSOR_SUMMARY -->
> [!NOTE]
> **Low Risk**
> Mechanical version-string and CI path updates only; no purchase, auth,
or SDK behavior changes.
> 
> **Overview**
> Bumps the repo from **10.19.0** to **10.20.0-SNAPSHOT** so `main`
targets the next development cycle after the release.
> 
> **Version identifiers** are updated in `.version`, `gradle.properties`
(`VERSION_NAME`), and `Config.frameworkVersion` in the purchases module.
> 
> **Consumer-facing samples and test apps** (Magic Weather, Magic
Weather Compose, Custom Entitlement Computation, SDK size testing) now
resolve `com.revenuecat.purchases` artifacts at **10.20.0-SNAPSHOT** via
their `libs.versions.toml` files.
> 
> **CI docs deploy** syncs Dokka output to
`s3://purchases-docs/android/10.20.0-SNAPSHOT` instead of `10.19.0`.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
741dfa78ed875be20c2df79abf8ce3bb171b64ab. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->