Update Scala to 2.13.12

[lefou]

No description provided.

[lefou]

Build fails with a strage error message:

 -------------------------------- Running Tests --------------------------------
[info] compiling 1 Scala source to /home/runner/work/mill/mill/out/integration/feature/scoverage/fork/workspaceDir.dest/out/mill-build/compile.dest/classes ...
[info] done compiling
1 targets failed
core[2.13.11].scoverageToolsClasspath 
Resolution failed for 1 modules:
--------------------------------------------
  org.scoverage:scalac-scoverage-plugin_2.13.12:2.0.10 
	not found: /home/runner/.ivy2/local/org.scoverage/scalac-scoverage-plugin_2.13.12/2.0.10/ivys/ivy.xml
	not found: https://repo1.maven.org/maven2/org/scoverage/scalac-scoverage-plugin_2.13.12/2.0.10/scalac-scoverage-plugin_2.13.12-2.0.10.pom

--------------------------------------------

For additional information on library dependencies, see the docs at
https://mill-build.com/mill/Library_Dependencies.html

X mill.integration.ScoverageTests.test 23340ms 
  utest.AssertionError: assert(eval("core[2.13.11].scoverage.xmlReport"))
    utest.asserts.Asserts$.assertImpl(Asserts.scala:30)
    mill.integration.ScoverageTests$.$anonfun$tests$2(ScoverageTests.scala:10)

Why strange? Because the project under test is configured to use Scala version 2.13.11 and Scoverage 2.0.10, but the error message indicates Scala version 2.13.12

[lefou]

Ah, scoverage reporting needs to run in the Mill classpath, hence it must be resolved with the Scala version used to run Mill.