RFR: 7541: Switch to using 2021-09 by default

[Marcus Hirt]

This is the platform that Oracle will use when releasing, so might as well make it the default.

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Commit messages:
 - 7541: Switch to using 2021-09 by default

Changes: https://git.openjdk.java.net/jmc/pull/371/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=371&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7541
  Stats: 6 lines in 1 file changed: 3 ins; 3 del; 0 mod
  Patch: https://git.openjdk.java.net/jmc/pull/371.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/371/head:pull/371

PR: https://git.openjdk.java.net/jmc/pull/371