RFR: 8310591: Missing `@since` tags in java.lang.foreign

[Per Minborg]

This PR adds missing  `@Since` and `@PreviewFeature` annotations to some of the Panama FFM entities.

This PR should also be back-ported to 21.

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Commit messages:
 - Add @Since and @PreviewFeature where missing

Changes: https://git.openjdk.org/jdk/pull/14611/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14611&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310591
  Stats: 8 lines in 3 files changed: 8 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/14611.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14611/head:pull/14611

PR: https://git.openjdk.org/jdk/pull/14611