`mcr.microsoft.com/devcontainers/base:ubuntu` points to `jammy` & major version release

📢 We have released a new major version for the [base-ubuntu](https://github.com/devcontainers/images/tree/main/src/base-ubuntu) image (ie [mcr.microsoft.com/devcontainers/base](https://github.com/devcontainers/images/blob/main/src/base-ubuntu/history/1.0.0.md)).

ℹ️ Now the `mcr.microsoft.com/devcontainers/base:ubuntu` points to the latest LTS release (ie. `jammy`)

Hence, if you previously used `mcr.microsoft.com/devcontainers/base:ubuntu` with the hope to use the `focal` variant. It needs to be updated to `mcr.microsoft.com/devcontainers/base:focal` instead.

ℹ️ Usually, the dev configuration files reference the images by pinning to a major version (details [here](https://github.com/devcontainers/images/tree/main/src/base-ubuntu#using-this-image)). In this case, the major version has bumped from `0` to `1`

Dev configuration files referencing to the old image tag (ie `0-${VARIANT}`) needs to be updated as below :point_down:

- 1-ubuntu
- 1-ubuntu22.04
- 1-ubuntu-22.04
- 1-jammy
- 1-ubuntu20.04
- 1-ubuntu-20.04
- 1-focal
- 1-ubuntu18.04
- 1-ubuntu-18.04
- 1-bionic