RFR: 8310997: missing @since tags in jdk.httpserver

[Darragh Clarke]

added missing `@since` tag to 4 files in jdk.httpserver

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Commit messages:
 - added @since to classes in jdk.httpserver

Changes: https://git.openjdk.org/jdk/pull/14755/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14755&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310997
  Stats: 12 lines in 4 files changed: 8 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/14755.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14755/head:pull/14755

PR: https://git.openjdk.org/jdk/pull/14755