Integrated: 8285: Documentation fix

[Marcus Hirt]

The documentation fails to mention that both JDK 17 and 21 are now needed in the toolchain.

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Commit messages:
 - 8285: Documentation fix

Changes: https://git.openjdk.org/jmc/pull/599/files
  Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=599&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-8285
  Stats: 22 lines in 1 file changed: 18 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jmc/pull/599.diff
  Fetch: git fetch https://git.openjdk.org/jmc.git pull/599/head:pull/599

PR: https://git.openjdk.org/jmc/pull/599