RFR: 7378: Update platform to 2021-06

[Marcus Hirt]

...and removing old ones.

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Commit messages:
 - Fix
 - Various fixes
 - Updating Spotbugs and Tycho
 - Another unused import
 - Unused import
 - Unrelated - fixing writer classpath, since tests were moved out into separate bundle
 - Removing old platforms
 - 7378: Update platform to 2021-06

Changes: https://git.openjdk.java.net/jmc/pull/301/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=301&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7378
  Stats: 365 lines in 34 files changed: 60 ins; 242 del; 63 mod
  Patch: https://git.openjdk.java.net/jmc/pull/301.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/301/head:pull/301

PR: https://git.openjdk.java.net/jmc/pull/301