docs: contributing instructions regarding versioning are out of date

[hraftery]

The README on versioning an exercise seems to reflect the reality prior to "Reopening the Problem Specifications repo": exercism/problem-specifications#1674.

From what I can see, versioning is still expected but:

• Exercises based on a canonical-data.json in problem-specifications should use its version plus a serial number.

is no longer true and what was the fallback:

• Exercises that are not based on canonical-data.json should use version 0.1.0 plus a serial number.

is the true and only option.

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[petertseng]

I think at this point we should make the serial be the only component, then. there's no point to any other components like 0.1.0 or 1.0.0.