build: remove legacy Java 9+ sbt plugin references

[He-Pin]

Motivation

Following apache/pekko#3075 which renamed the Jdk9 sbt plugin to Jdk21 (since Java 17 is now the project baseline and JDK 9 features are part of the standard build), the AutomaticModuleName sbt helper and --add-opens JVM options inherited from the Java 9 module-system era are stale references in this project. They add noise to the build and mislead developers about the actual Java baseline.

Modification

• Delete project/AutomaticModuleName.scala (JPMS Automatic-Module-Name manifest helper).
• Remove AutomaticModuleName.settings(...) calls and the inline "Automatic-Module-Name" -> "..." manifest entry from build.sbt.
• Remove --add-opens=java.base/...=ALL-UNNAMED JVM options (both the single += form and the Seq(...) form) from build.sbt.
• Update stale // JDK 11: ... comments to // project baseline is Java 17: ... (aligned with refactor: rename Jdk9 sbt plugin to Jdk21 pekko#3075).

Result

Build definitions no longer reference legacy Java 9+ module-system helpers or JVM options. Project baseline remains Java 17; no runtime behavior change for end users. Downstream consumers relying on the previous Automatic-Module-Name manifest entries or on --add-opens to access jdk.internal.misc from test code should note the removal.

Tests

• sbt "show name" — success (build.sbt parses and loads cleanly for all modules).
• Parenthesis balance check — passes (open == close in stripped AST).
• git diff --check — clean.

References

• refactor: rename Jdk9 sbt plugin to Jdk21 pekko#3075 — same cleanup pattern applied to the core pekko build.

[He-Pin]

Closing — the PR scope was incorrect. AutomaticModuleName (JPMS manifest) and --add-opens (reflective library support) are functional Java 9+ features, not stale references. A correct cleanup aligned with apache/pekko#3075 should be limited to the Jdk9 sbt plugin naming (which this project doesn't have) and stale jdk9 comments. Apologies for the noise.