[ci] Update linux image name from bionic to focal

[cclauss]

Reduce confusion by using the correct Ubuntu release name. Currently, the tag is bionic but the image is focal.

-  bionic:
+  focal:
     docker:
       - image: emscripten/emscripten-ci:focal

https://releases.ubuntu.com

Should upgrading to Jammy 22.04 or Noble 24.04 be considered?

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Also, resolve four problems raised by the VS Code extension for CircleCI:

[sbc100]

Does the old version not work?

[cclauss]

CircleCI flags this as an incorrect data type error which means it could easily stop working in future releases.

[sbc100]

Interesting, but they don't show it as a config error. I wonder why not? LGTM to this part either way.

[cclauss]

https://circleci.com/docs/vs-code-extension-overview/#config-helper is in-context help..