GA Migration Java PostCommit Nexmark Dataflow

[andoni-guzman]

This PR is part of the migration from jenkins to github actions.

This PR contains this tests

Java PostCommit Nexmark Dataflow - Java 8 Java PostCommit Nexmark Dataflow Direct - Java 8, Java 11 and Java 17

2 Files were added to the workflows folder of github actions - job-postcommit-java-nexmark-dataflow and - job-postcommit-java-nexmark-dataflow V2 4 Files were deleted, this files were the groovy files for jenkins - Dataflow - Dataflow V2 Java 8 - Dataflow V2 Java 11 - Dataflow V2 Java 17 Jenkins readme was modified deleting migrated tests. CI.md was modified adding the new GA workflows

Waiting for this PR before merge https://github.com/apache/beam/pull/22703

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