RFR: 7903462: JMH: Upgrade pre-integration workflows

[Aleksey Shipilev]

JDK 20 was released, and we need to start tracking 21-ea.

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jmh/pull/101/files
 Webrev: https://webrevs.openjdk.org/?repo=jmh&pr=101&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903462
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jmh/pull/101.diff
  Fetch: git fetch https://git.openjdk.org/jmh.git pull/101/head:pull/101

PR: https://git.openjdk.org/jmh/pull/101