RFR: 8308281: Java snippets in the FFM API need to be updated

[Per Minborg]

As the API has improved over the recent releases, not all `{@snippet ..}` sections have been kept in sync.

This PR suggests all snippets used should be verified against real code that is placed in a new `snippet-files` folder and erroneous snippets are updated.

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Commit messages:
 - Reformat
 - Remove file
 - Add a snippet-files class with snippets

Changes: https://git.openjdk.org/jdk/pull/14030/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14030&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308281
  Stats: 714 lines in 7 files changed: 679 ins; 1 del; 34 mod
  Patch: https://git.openjdk.org/jdk/pull/14030.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14030/head:pull/14030

PR: https://git.openjdk.org/jdk/pull/14030