Upgrade to Scala 2.12.9

[xerial]

• Scala.js support Scala 2.12.9 support scala-js/scala-js#3738
  • It hits a bug of Scala compiler Spurious "cannot be checked at runtime" in latest 2.12.x (regression wrt. 2.12.8) scala/bug#11534 so we probably need to wait Scala 2.12.10, which will have this fix More accurate outer checks in patterns scala/scala#8261

[codecov]

Codecov Report

Merging #576 into master will decrease coverage by 0.86%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #576      +/-   ##
==========================================
- Coverage   82.95%   82.09%   -0.87%     
==========================================
  Files         226      226              
  Lines        8284     8371      +87     
  Branches      608      613       +5     
==========================================
  Hits         6872     6872              
- Misses       1412     1499      +87

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[xerial]

It seems we should wait Scala 2.12.10 because of this string interpolation regression scala/bug#11665 in https://github.com/scala/bug/milestone/102. Spark will also wait for 2.12.10 apache/spark#25404