Emit a warning when using k6 convert to point to new har-to-k6converter

[markjmeier]

As the title says,k6 convert should emit a warning to point users to the new har-to-k6 tool

[ppcano]

If the har-to-k6 tool will be the recommended tool, the docs https://docs.k6.io/docs/session-recording-har-support should also be updated.

[mstoykov]

I reopen this as it was long time ago reverted with #1268

[mstoykov]

I am of the opinion that the rational back then was that the lacking functionality was somehow particularly useful and we can't remove it without it.

Given that at least some of the features are still not implemented 2 years+ later - they likely won't be.

I would also argue that given that we haven't fixed anything around the har converter all this time -and haven't gotten any more reports - it is either not used or is perfect. The later seems unlikely.

I would propose that we:

1. Mark is a deprecated (this issue).
2. Wait for few releases - we can do as many as we want but I would argue more then 2 less than 6. So let's say the maximum 5 releases.
   3.1 If there is feedback that this is used and is needed and har-to-k6 does not work well enough for this uses case. We would have good arguments for the changes in har-to-k6.
   3.2. If there isn't any feedback - it is very likely this just isn't used. In which case after those releases we drop in the next one.
3. We can always make k6 convert into its own tool that has the same code but isn't part of k6 if it turns out there is interest.