RFR: 8317874: Add @sealedGraph to StringTemplate.Processor.Linkage

[Per Minborg]

This PR proposes to Add @sealedGraph to `StringTemplate.Processor.Linkage`

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Commit messages:
 - Add @sealedGraph to StringTemplate.Processor.Linkage

Changes: https://git.openjdk.org/jdk/pull/16141/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16141&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317874
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/16141.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16141/head:pull/16141

PR: https://git.openjdk.org/jdk/pull/16141