RFR: 8284702: Add @since for java.time.LocalDate.EPOCH

[Glavo]

`java.time.LocalDate.EPOCH` was introduced in Java 9, but there is no corresponding `@since` in the javadoc. The absence of `@since` makes it impossible for IDEs to check for misuse of it, it may be misused by users targeting Java 8 and crash at runtime.

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Commit messages:
 - 8284702: Add @since for java.time.LocalDate.EPOCH

Changes: https://git.openjdk.java.net/jdk/pull/8192/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8192&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284702
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8192.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8192/head:pull/8192

PR: https://git.openjdk.java.net/jdk/pull/8192