[FLINK-12461] Document Scala 2.12.x situation

[aljoscha]

What is the purpose of the change

Scala versions after 2.12.8 are not binary compatible with previous versions, we link to the relevant release notes and point users towards building their own release using a more recent version.

It's a pure documentation change, no testing needed.

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[sjwiesman]

Thanks for taking the time, I made a suggestion to be a little more clear with our language but the basic content makes sense. Also +1 to Chesnay's comment. Feel free to address when merging :)

[aljoscha]

Thanks for the reviews!

[aljoscha]

merged