RFR: 8309937: Add @sealedGraph for some Panama FFM interfaces

[Per Minborg]

This PR proposes adding `@sealedGraph` to `GroupLayout` so the latter will be rendered in the javadocs the same way the other layouts that has public sub-interfaces.

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Commit messages:
 - Add @sealedGraph to GroupLayout

Changes: https://git.openjdk.org/jdk/pull/14449/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14449&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309937
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/14449.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14449/head:pull/14449

PR: https://git.openjdk.org/jdk/pull/14449