Uses the new `MediaFile.persist()` [API](https://github.com/matrix-org/matrix-rust-sdk/pull/2789) to cache voice messages. This allows to close the `MediaFile` handle after use and keeping the file. Also disables rust sdk caching for voice messages as we'll use the app's cache dir for that purpose. Fixes: https://github.com/vector-im/element-meta/issues/2175