CI updates: JDK 17, Scala 2.13.6, drop Scala 3.0.0-RC3

[SethTisue]

No description provided.

[SethTisue]

[error] Test scala.compat.java8.runtime.LambdaDeserializerTest.cached failed: java.lang.RuntimeException: java.lang.IllegalArgumentException: too many arguments, took 0.004 sec
[error]     at scala.compat.java8.runtime.LambdaDeserializerTest.reconstitute(LambdaDeserializerTest.java:147)
[error]     at scala.compat.java8.runtime.LambdaDeserializerTest.cached(LambdaDeserializerTest.java:79)
[error]     at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[error]     at jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
[error]     at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[error]     at java.lang.reflect.Method.invoke(Method.java:568)
[error]     ...
[error] Caused by: java.lang.IllegalArgumentException: too many arguments
[error]     at java.lang.invoke.LambdaMetafactory.altMetafactory(LambdaMetafactory.java:511)
[error]     at scala.compat.java8.runtime.LambdaDeserializer$.makeCallSite$1(LambdaDeserializer.scala:94)
[error]     at scala.compat.java8.runtime.LambdaDeserializer$.deserializeLambda(LambdaDeserializer.scala:112)
[error]     at scala.compat.java8.runtime.LambdaDeserializer.deserializeLambda(LambdaDeserializer.scala)
[error]     at scala.compat.java8.runtime.LambdaDeserializerTest.reconstitute(LambdaDeserializerTest.java:145)
[error]     ... 38 more

this is scala/bug#12419 , which has been fixed for Scala 2.12.15, which hasn't been released yet

leaving this as draft for now — we can revisit it once 2.12.15 comes out

[SethTisue]

(the CI failure is expected — it's because genjavadoc isn't published for the Scala nightly that the temporary commit uses. the one we care about at the moment is https://app.travis-ci.com/github/scala/scala-java8-compat/jobs/524773023, and it's green)

[SethTisue]

I've set a personal reminder to reopen this once 2.12.15 is out.

[SethTisue]

we need to land #242 first

[SethTisue]

I ended up just folding this stuff into #242