RFR: 8311001: missing @since info in jdk.net

[Darragh Clarke]

Added missing `@since` tag to `Sockets.java`

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Commit messages:
 - updated copyright year
 - added missing @since tag to sockets class

Changes: https://git.openjdk.org/jdk/pull/14756/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14756&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311001
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14756.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14756/head:pull/14756

PR: https://git.openjdk.org/jdk/pull/14756