RFR: 8297152: Add a @sealedGraph tag to ZoneId

[Naoto Sato]

Adding a `@sealedGraph` tag to `ZoneId` class.

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Commit messages:
 - 8297152: Add a @sealedGraph tag to ZoneId

Changes: https://git.openjdk.org/jdk/pull/11195/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11195&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297152
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11195.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11195/head:pull/11195

PR: https://git.openjdk.org/jdk/pull/11195