[FLINK-40764][benchmarks] Add a PR template with AI disclosure

.github/PULL_REQUEST_TEMPLATE.md

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+Licensed to the Apache Software Foundation (ASF) under one
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+Thank you for contributing to Apache Flink benchmarks.
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+
+## Purpose of the change
+
+<!-- Link the Jira issue, if applicable, and explain the problem this change addresses. -->
+
+## Brief change log
+
+<!-- Summarize the changes and identify the affected benchmarks or tooling. -->
+
+## Verifying this change
+
+<!--
+Describe the checks performed and their results. For benchmark changes, include
+benchmark results and the execution context (commands, versions, and environment).
+For changes that do not affect benchmarks, describe the relevant checks instead.
+-->
+
+## Impact and documentation
+
+<!--
+Describe any effects on benchmark behavior, result comparability, or dependencies,
+and any corresponding documentation updates. If none apply, state "Not applicable".
+-->
+
+---
+
+##### Was generative AI tooling used to co-author this PR?
+
+<!--
+If generative AI tooling has been used in the process of authoring this PR, please
+change the checkbox below to `[X]` and replace the placeholder in the "Generated-by"
+line with the tool name and version. Otherwise remove the "Generated-by" line.
+See the ASF Generative Tooling Guidance for details:
+https://www.apache.org/legal/generative-tooling.html
+
+You are responsible for the quality and correctness of every change in this PR
+regardless of the tooling used. Low-effort AI-generated PRs will be closed. See
+https://flink.apache.org/how-to-contribute/contribute-code/#ai-assisted-contributions
+for the full guidance.
+-->
+
+- [ ] Yes (please specify the tool below)
+
+Generated-by: [Tool Name and Version]