Integrated: Update maven.yaml

[Leonid Kuskov]

Update maven.yaml - bump up the JDK version to 21

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Commit messages:
 - Update maven.yaml

Changes: https://git.openjdk.org/asmtools/pull/77/files
  Webrev: https://webrevs.openjdk.org/?repo=asmtools&pr=77&range=00
  Stats: 3 lines in 1 file changed: 0 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/asmtools/pull/77.diff
  Fetch: git fetch https://git.openjdk.org/asmtools.git pull/77/head:pull/77

PR: https://git.openjdk.org/asmtools/pull/77