RFR: 8305758: Update the man page to indicate -i/--generate-file is deprecated

[Lance Andersen]

Hi all,

Please review this MR which updates the jar man page to to indicate that  --generate-index/-I are deprecated and may be removed in a future release

Support for JAR Index is being removed via JDK-8302819.

-------------

Commit messages:
 - Update the man page to indicate -i/--generate-file is deprecated

Changes: https://git.openjdk.org/jdk/pull/13400/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13400&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305758
  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/13400.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13400/head:pull/13400

PR: https://git.openjdk.org/jdk/pull/13400