Update Devcontainer Dockerfile

This PR updates the .devcontainer Dockerfile which was no longer
working. Attempting to build an image from the Dockerfile would result
in an error:

```
E: The repository 'http://archive.ubuntu.com/ubuntu kinetic Release' does not have a Release file.
```

AFAIU, this is because the 22.04 release is no longer supported. I
changed the base image to the current LTS release, which should remain
supported until 2029.

In addition, the Dockerfile is also modified to install `emscripten` APT
package. While this is not the recommented installation method from the
Emscripten docs, it is much more convenient and works enough to build
the simulator using the resulting Docker image.

.devcontainer/Dockerfile

@@ -1,6 +1,4 @@
-FROM ubuntu:22.10
-
-# TODO: install emscripten (https://emscripten.org/docs/getting_started/downloads.html)
+FROM ubuntu:24.04
 
 # TODO: Clean this up once buildkit is supported gracefully in devcontainers
 # https://github.com/microsoft/vscode-remote-release/issues/1409
@@ -27,7 +25,9 @@ RUN apt-get update \
     # ca certs need to be available for fetching git submodules
     ca-certificates \
     # python is used to convert binaries to uf2 files
-    python3 python-is-python3
+    python3 python-is-python3 \
+    # emscripten for building simulator
+    emscripten
 
 # Download and verify both x86-64 and aarch64 toolchains. This is unfortunate and
 # slows down the build, but it's a clean-ish option until buildkit can be used.
@@ -47,4 +47,4 @@ RUN /bin/sh -c 'set -ex && \
     fi'
 
 RUN rm $X86_64_TOOLCHAIN_FILENAME
-RUN rm $AARCH64_TOOLCHAIN_FILENAME
+RUN rm $AARCH64_TOOLCHAIN_FILENAME