Add milestones workflow for Merged PRs

[NouemanKHAL]

What does this PR do?

Adds a github workflow that runs on merged PRs in master and [0-9]+.[0-9]+.x release branches to automatically assign them to the current_milestone of their branch.

Motivation

Make it easy to track each PR and its corresponding agent release version

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[github-actions]

⚠️ Recommendation: Add qa/skip-qa label

This PR does not modify any files shipped with the agent.

To help streamline the release process, please consider adding the qa/skip-qa label if these changes do not require QA testing.

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: cebc642e70

ℹ️ About Codex in GitHub

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• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[chatgpt-codex-connector]

Trigger milestone updates from a write-capable event

This workflow is wired to pull_request events, but it performs a write operation with gh issue edit later in the job; for PRs coming from forks (including Dependabot), GITHUB_TOKEN is read-only on pull_request, so merged external PRs will fail to set milestones. In practice this means a non-trivial share of merged PRs will never be milestoned even though the workflow runs.

Useful? React with 👍 / 👎.