Provisioning: stop installing the stale r29-beta2 preview NDK

NDK r29 shipped in October 2025 and r30 is now the latest,
while Google lists no r31 preview yet. Point the preview slot
at the latest NDK, which the scripts treat as nothing extra
to install, the same way the nightly slots already do. The
preview nightly configuration stays idle until a new preview
NDK appears.

Pick-to: 6.12
Task-number: QTBUG-143599
Change-Id: Ie8290a12648c350e522c2b3e874a78371cdde827
Reviewed-by: Elias Toivola <elias.toivola@qt.io>