RFR: 8254704: Add missing @since tag to BodyPublishers::concat

[Daniel Fuchs]

Please review a trivial change that adds a missing `@since 16` tag to the API documentation of the new
`BodyPublishers::concat` method. I missed that in my changes that was integrated yesterday - and that was unfortunately
missed in the review and CSR as well.

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Commit messages:
 - 8254704: Add missing @since tag to BodyPublishers::concat

Changes: https://git.openjdk.java.net/jdk/pull/638/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=638&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254704
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/638.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/638/head:pull/638

PR: https://git.openjdk.java.net/jdk/pull/638