Move Java Example distroless Dataflow to Java21 precommit

[Abacn]

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[Abacn]

Tested: https://github.com/apache/beam/actions/runs/15115828814/job/42486135065 https://github.com/apache/beam/actions/runs/15115830393/job/42486140126

[Abacn]

both precommit tasks are back greeen

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[Abacn]

R: @akashorabek @sjvanrossum

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