Bump PostCommit Java DataflowV1 timeout to 6h

[Abacn]

It is consistently timing out

Please add a meaningful description for your change here

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[Abacn]

Fix timeout seen in #30081

R: @johnjcasey

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[Abacn]

R: @damccorm