Chore: Automate updating default versions of VM images

[KyleTryon]

Currently, the default image information for each of the VM executors is set statically. As proposed in #264 this may be automated with tools like Renovate app.

Renovate app can use regex to identify packages in any file and pair it with a source.

The blocking issue, is we currently have no acceptable source for VM images suitable for Renovate app.

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Example of static VM image
https://github.com/CircleCI-Public/visual-config-editor/blob/fd4fd3a2a207b6db471c73100481c0a9294e4c33/src/mappings/components/ExecutorMapping.tsx#L54