Fix BuildSpanAction deserialization logic

[nikita-tkachenko-datadog]

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What does this PR do?

Fixes custom deserialisation logic of the BuildSpanAction class.
The version field was added to the v1 of the converter erroneously.
The data written by the original converter v1 (introduced in the plugin v6.0.2) does not contain version field, so when the updated converter code tries to deserialize this field, it fails.
The versioned converters are supposed to be immutable, the code was changed by mistake.

The bug was introduced in PR#387 Implement submitting in-progress pipelines data.

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