Add java 21 and 25 to build matrix

[Philippus]

[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.

• https://github.com/scala/scala/pull/11028
• https://github.com/scala/scala-async/blob/0eaee561952da486a7949d1d9804f145d2a4a9e0/build.sbt#L14
• 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: https://github.com/scala/scala-async/pull/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 https://github.com/scala/scala-async/pull/380.