Use Java 8 to build release artifacts

[lefou]

No description provided.

[ice1000]

What's the rationale?

[lefou]

Before this PR, we published with Java 9. Although we explicitly select the target as Java 8, some binary incompatibilities sneaked in all of our current releases. So, I think it's safer to just use Java 8 for compiling the release versions.

See the output of this experimental integration of MiMa in the mill build:

09:59:20 ⌁ lefou:~/work/opensource/mill mima+* 130 ± mill -j 0 main.__.mimaReportBinaryIssues
Using experimental parallel evaluator with 16 threads
[#07] [281/306] main.core.compile 
[#11] Scanning 79 classfiles for binary compatibility in /home/lefou/work/opensource/mill/out/main/api/compile/dest/classes ...
[#03] [282/306] scalalib.api.localClasspath 
[#11]  * static method transferTo(java.io.OutputStream)Long in class mill.api.DummyInputStream does not have a correspondent in current version
   filter with: ProblemFilters.exclude[DirectMissingMethodProblem]("mill.api.DummyInputStream.transferTo")
[#11]  * static method readNBytes(Array[Byte],Int,Int)Int in class mill.api.DummyInputStream does not have a correspondent in current version
   filter with: ProblemFilters.exclude[DirectMissingMethodProblem]("mill.api.DummyInputStream.readNBytes")
[#11]  * static method readAllBytes()Array[Byte] in class mill.api.DummyInputStream does not have a correspondent in current version
   filter with: ProblemFilters.exclude[DirectMissingMethodProblem]("mill.api.DummyInputStream.readAllBytes")

You can see that comparing the HEAD of 0.9.x branch with the 0.9.9 tag, some methods only defined in Java 9 sneak into our API. This happens when I run (and compile) mill with Java 8.

[ice1000]

Thanks for the explanation!

[lolgab]

Glad that the mill-mima plugin is already creating some value 😍