Require JDK17+ for development

[haydenbaker]

Background

• Given JDK8 is largely being moved away from, this is a good first step in that direction
• However, for backwards compatibility, we will still target 8 for all current modules
  • This means existing modules cannot use newer features, but new modules (like docgen) can use newer language versions
  • Speaking optimistically, some day we can remove explicit compatibility and just rely on the toolchain

Testing

• ./gradlew clean build integ publish
• Set global Java to <17, and run again, does not build and shows error

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