Warn that we no longer support FFmpeg 3.x

[mikeboers]

I'm not sure how, but it would be really nice to do this. See #603 and #604.

[DurandA]

Also, it would be nice to update the docs for Ubuntu >= 18.04 LTS as 18.04 comes with FFmpeg 3.4.6.

[mikeboers]

OOookay. I finally feel compelled that I need to do something. I'm thinking at a minimum adding a compile-time error that checks against the library versions to absolutely assert it is building against the version we think it is.

[mikeboers]

If it is trivial, e.g. just a couple of constants not working, I'll re-add 3x support. But I don't think it is trivial.

[jlaine]

I'd say goodbye FFmpeg 3.x, it's hard enough maintaining the project as is..

[mikeboers]

@jlaine Thanks for the vote.

I'm just trying to figure out the macOS tests on GA, and then this warning will be rolled out.

[jlaine]

I'm closing this, doesn't seem worth the hassle