RFR: 8308281: Java snippets in the FFM API need to be updated [v5]

[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.
> 
> In this PR, it is suggested duplicating code in the `Snippets.java` class and in the JavaDocs. The benefit of this is that code is directly visible in the code and not only in the generated javadoc.
> 
> Another thing to think about is if there should be on single `Snippets.java` class or separate ones for each FFM class.

Per Minborg has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains seven commits:

 - Merge master
 - Remove hybrid snippets
 - Use hybrid snippets for Arena
 - Fix according to comments
 - Reformat
 - Remove file
 - Add a snippet-files class with snippets

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Changes: https://git.openjdk.org/jdk/pull/14030/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14030&range=04
  Stats: 750 lines in 7 files changed: 689 ins; 4 del; 57 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