Update Emscripten version in Qt+WASM Docker containers

The version of Emscripten SDK used in WebAssembly docker containers should be updated to be compatible with the Qt version used inside the container. https://doc.qt.io/qt-6/wasm.html states some minimum version that we probably should follow.

At least with Qt6.3.1 + WASM container the old version leads to some problems

• `emrun` doesn't work inside the container
• build errors might occur

To be confirmed, if similiar problems exist with Qt6.3.0 and older versions

root cause is the mapping `emsdk_version = "3.0.0" if branch == qt_branch_630 else "2.0.14"` in the image creation.