Provide better maintainability of version-specific assets

There are a number of Dockerfiles in this repo that install specific versions of software ([example](https://github.com/dotnet/dotnet-buildtools-prereqs-docker/blob/9a8452435e6c19aa6c5dcd67cc2e207dc7d86ba6/src/ubuntu/20.04/webassembly/amd64/Dockerfile#L49-L59)). This leads to a maintenance problem as those versions eventually become EOL or exposed to known vulnerabilities. In order to reduce the amount of maintenance necessary for this repo, there needs to be some improvements here.