add a JDK 15 build

[SethTisue]

It's time!

(Aside: while I'm doing this, I think I would like to rename the JDK 8 Jenkins jobs to have -jdk8- in the name, since on 2.13.x the -jdk11- build is now the "main" build — or perhaps there isn't any "main" build anymore, since we now have at least one repo that fails in all three builds (fs2 fails in JDK 8, shapeless-java-records fails on 8 and 11, and several repos still fail on JDK 14 just because the maintainers haven't dealt with it). There is still a "main" build in the sense of "JDK version I typically test on", but it's no longer consistent across Scala versions which one that is: 2.11.x and 2.12.x remains 8, but on 2.13.x I do 11 first, and check 8 later.)

[SethTisue]

• on all 3 behemoths:
  • sudo apt-get install adoptopenjdk-15-hotspot
  • cd /usr/lib/jvm; sudo ln -s adoptopenjdk-15-hotspot-amd64 java-15-openjdk-x64
• rename JDK 8 jobs to include -jdk8- in name
• rename jobs from -jdk14- to -jdk15- and change JDK version in job configs from 14 to 15
• update report/Report.scala on both 2.12.x and 2.13.x branches
• unfreeze shapeless-java-records
• update community build wiki
• make sure both jobs have at least a green core
• update https://docs.scala-lang.org/overviews/jdk-compatibility/overview.html
• open a separate ticket on gradually getting more projects green

[SethTisue]

[scala-xml] Using sbt version: 0.13.18
[scala-xml:error] Unrecognized VM option 'CMSClassUnloadingEnabled'
[scala-xml:error] Error: Could not create the Java Virtual Machine.
[scala-xml:error] Error: A fatal exception has occurred. Program will exit.

I could try to figure out where this is coming from, but another option would be to -XX:+IgnoreUnrecognizedVMOptions it

[SethTisue]

I solved the CMSClassUnloadingEnabled problem with 71bcb48

[SethTisue]

153 green at https://scala-ci.typesafe.com/view/scala-2.12.x/job/scala-2.12.x-jdk15-integrate-community-build/1135/
176 green at https://scala-ci.typesafe.com/job/scala-2.13.x-jdk15-integrate-community-build/1251/

declaring victory

[SethTisue]

PR updating JDK compat page: scala/docs.scala-lang#1793

[SethTisue]

followup ticket about getting even more repos green: #1240