RFR: 8260517: implement Sealed Classes as a standard feature in Java [v6]

[Vicente Romero]

> Please review this PR that intents to make sealed classes a final feature in Java. This PR contains compiler and VM changes. In line with similar PRs, which has made preview features final, this one is mostly removing preview related comments from APIs plus options in test cases. Please also review the related [CSR](https://bugs.openjdk.java.net/browse/JDK-8265090)
> 
> Thanks
> 
> note: this PR is related to [PR-3528](https://github.com/openjdk/jdk/pull/3528) and must be integrated after it.

Vicente Romero has updated the pull request incrementally with one additional commit since the last revision:

  restoring jdk.internal.javac.PreviewFeature.Feature.SEALED_CLASSES, it is needed by the build

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3526/files
  - new: https://git.openjdk.java.net/jdk/pull/3526/files/2744f615..304fd76a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3526&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3526&range=04-05

  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3526.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3526/head:pull/3526

PR: https://git.openjdk.java.net/jdk/pull/3526