linux: drop ubuntu 18.04 devcontainer

[iMichka]

Ubuntu 18.04 is EOL now, so stop building containers for it.

Also adjust some dates in the documentation

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

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[iMichka]

Is someone still using .devcontainer/ubuntu18.04/devcontainer.json ?

[Bo98]

We should add a .docker-deprecate message to the image (and the org.opencontainers.image.support.end-of-support label) beforehand to announce this removal for the next minor release or something. a098519

For 16.04 we announced the deprecation in 3.6.0 (or well technically 3.6.1 as we just missed it and called it close enough) and removed it in 4.0.something (a little late because 4.0.0 was hectic).

[MikeMcQuaid]

Is someone still using .devcontainer/ubuntu18.04/devcontainer.json ?

No, please feel free to remove.

[MikeMcQuaid]

Thanks @iMichka! Have removed Docker image deprecation, pulled that into #18387.

[iMichka]

Thanks