And make it easier for renovate to keep it up to date.

The old `1` refers to the major version of the image itself.
The old `18` refers to the node version in that image.

Here is a list of tag for comparison:

https://mcr.microsoft.com/v2/devcontainers/typescript-node/tags/list

The new shorthand `20` refers to "the latest" node 20 variant, and the docker sha pin ensures it is reproducible.