RFR: JDK-8309883: no @since info in com.sun.tools.javac package-info.java,Main.java

[Jonathan Gibbons]

Please review a trivial update to add some long-overdue `@since` tags to public API in `com.sun.tools.javac`.

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Commit messages:
 - JDK-8309883: no @since info in com.sun.tools.javac package-info.java, Main.java

Changes: https://git.openjdk.org/jdk/pull/14431/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14431&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309883
  Stats: 4 lines in 2 files changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/14431.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14431/head:pull/14431

PR: https://git.openjdk.org/jdk/pull/14431