docs: update supported platforms and test execution info

- List officially supported platforms tested in CI:
  Ubuntu 22.04 LTS, Ubuntu 24.04 LTS (x86_64 and ARM64), macOS latest
- Clarify that builds are tested on all platforms but full test suite
  only runs on ubuntu-latest
- Replace outdated Ubuntu 20.04+ reference with current CI matrix

This gives developers clear guidance on which platforms are officially
supported and continuously tested.

Author: dannywillems

website/docs/developers/getting-started.mdx

@@ -22,8 +22,13 @@ development environment and build OpenMina from source.
 
 ### System Requirements
 
-- **Operating System**: Ubuntu 20.04+ or Debian 11+ (other Linux distributions
-  may work but are not officially supported)
+- **Operating System**: Officially supported platforms (build tested in CI):
+  - Ubuntu 22.04 LTS
+  - Ubuntu 24.04 LTS (x86_64 and ARM64)
+  - macOS (latest)
+  - Other Linux distributions may work but are not officially supported
+  - Note: While we test compilation on all platforms, the full test suite is
+    only run on ubuntu-latest
 - **Memory**: At least 8GB RAM (16GB recommended)
 - **Storage**: At least 20GB free space
 - **Network**: Stable internet connection for downloading dependencies