RFR: 8317870: Add @sealedGraph to Thread.Builder

[Per Minborg]

This PR proposes to Add @sealedGraph to `Thread.Builder`.

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Commit messages:
 - Add @sealedGraph to Thread.Builder

Changes: https://git.openjdk.org/jdk/pull/16138/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16138&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317870
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/16138.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16138/head:pull/16138

PR: https://git.openjdk.org/jdk/pull/16138