Integrated: Merge jdk20

Wed Mar 22 14:53:42 UTC 2023

This PR merges the contents of `jdk20` into `master`. This means that JDK 20 will now become the "default" (and will be used as a basis for generation of binary snapshots).

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Commit messages:
 - Building/testing section is missing from README.md
 - Merge branch 'jdk20' into master_20
 - Add section on pre-built binaries to README
 - added a note on JDK version to use for Gradle build.

The merge commit only contains trivial merges, so no merge-specific webrevs have been generated.

Changes: https://git.openjdk.org/jextract/pull/115/files
  Stats: 0 lines in 0 files changed: 0 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jextract/pull/115.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/115/head:pull/115

PR: https://git.openjdk.org/jextract/pull/115