RFR: 7477: Removing old platforms

[Marcus Hirt]

This would be a change from our stated intention to support the last four releases, so we may want to hold off until 2021-03 to take this.

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Commit messages:
 - 7477: Removing old platforms

Changes: https://git.openjdk.java.net/jmc/pull/361/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=361&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7477
  Stats: 449 lines in 10 files changed: 0 ins; 448 del; 1 mod
  Patch: https://git.openjdk.java.net/jmc/pull/361.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/361/head:pull/361

PR: https://git.openjdk.java.net/jmc/pull/361