[jdk20] RFR: JDK-8298277: Replace "session" with "scope" for FFM access

[Per Minborg]

This PR proposes changing variable names from `session` to `scope`. The proposed changes are only for the FFM API and not other parts like the Vector API and test classes.

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Commit messages:
 - Replace session with scope for FFM access

Changes: https://git.openjdk.org/jdk20/pull/29/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk20&pr=29&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298277
  Stats: 131 lines in 19 files changed: 0 ins; 0 del; 131 mod
  Patch: https://git.openjdk.org/jdk20/pull/29.diff
  Fetch: git fetch https://git.openjdk.org/jdk20 pull/29/head:pull/29

PR: https://git.openjdk.org/jdk20/pull/29