[FLINK-14276][quickstarts] Scala quickstart compiles on JDK 11

[zentol]

Fixes an issue where a scala quickstart project would not compile on Java 9+. Same approach as in #6623.

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Last check on commit 5077c53 (Wed Oct 16 08:35:15 UTC 2019)

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CI report:

• 5077c53 : FAILURE Build

[tillrohrmann]

The change itself looks good. I was wondering whether we can only activate it if Java 11 is used?

[tillrohrmann]

Thanks for this fix @zentol. LGTM. +1 for merging.