fix JavaCompilerSpec with JDK 21

[xuwei-k]

suppress following message

Note: Annotation processing is enabled because one or more processors were found
  on the class path. A future release of javac may disable annotation processing
  unless at least one processor is specified by name (-processor), or a search
  path is specified (--processor-path, --processor-module-path), or annotation
  processing is enabled explicitly (-proc:only, -proc:full).
  Use -Xlint:-options to suppress this message.
  Use -proc:none to disable annotation processing.

[xuwei-k]

??? 🤔

Pull request submitted by xuwei-k
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Check if CLA is signed: https://www.lightbend.com/contribute/cla/scala/check/xuwei-k

from my local

$ curl https://www.lightbend.com/contribute/cla/scala/check/xuwei-k
{"user":"xuwei-k","signed":true,"version":"1.0","currentVersion":"1.0"}

[eed3si9n]

Thanks!