Add java 21 and 25 to build matrix

[Philippus]

No description provided.

[xuwei-k]

[error] Error while emitting Async.scala
[error] Unsupported class file major version 69

Scala 2.12.20 optimizer does not support JDK 25.

• [2.12.x] Upgrade asm for JDK25 support scala#11028

• scala-async/build.sbt

  Line 14 in 0eaee56

  ScalaModulePlugin.enableOptimizer,

• https://github.com/scala/sbt-scala-module/blob/689ddf1548431a11cb51843b3bd2c097d493d404/src/main/scala/ScalaModulePlugin.scala#L44-L51

[Philippus]

I've made a separate PR adding only java 21 here: #374

[SethTisue]

We will do 2.12.21, with JDK 25 support on the optimizer, before toooooo much longer, as per https://contributors.scala-lang.org/t/scala-2-12-21-release-planning/6753

[Philippus]

Closed in favour of #380.