Remove `enableTruncation` and `streamGenAiSpans` flag and decide on strategy for dropped large spans

[nicohrubec]

Still needs to be confirmed that we can do this but if so this also includes removing truncation altogether and coming up with a strategy for how to handle the issue more broadly. Alternatives:

• If span streaming is the default this is solved anyways and there is no need for truncation or this weird v1/v2 mixing anymore.
• If span streaming is still opt-in we can ask users to use it if they run into this issue with dropped spans because of size limits.
• Since we have had some time to test this properly we can also keep doing this v1/v2 envelope mixing but no longer guard it behind a flag.

[linear-code]

JS-2593