RFR: 7903489: JCStress: Upgrade pre-integration workflows

[Aleksey Shipilev]

We need to start testing 21-ea for upcoming release.

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Commit messages:
 - Drop JDK 11 as build target
 - Fix

Changes: https://git.openjdk.org/jcstress/pull/138/files
 Webrev: https://webrevs.openjdk.org/?repo=jcstress&pr=138&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903489
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jcstress/pull/138.diff
  Fetch: git fetch https://git.openjdk.org/jcstress.git pull/138/head:pull/138

PR: https://git.openjdk.org/jcstress/pull/138