[8.18] (backport #9376) ci: correct PR description format for update versions automated PRs

[mergify]

What does this PR do?

This PR fixes the description format of update versions automated PRs, e.g. #9339. The description after this PR here should appear like the following

---

The version file(s) changed
These files are used for picking the starting (pre-upgrade) or ending (post-upgrade) agent versions in upgrade integration tests.

The content is based on responses from https://www.elastic.co/api/product_versions and https://snapshots.elastic.co

The current update is generated based on the following requirements:

.package-version

{
  "version": "9.2.0-SNAPSHOT",
  "build_id": "9.2.0-8077f0d2",
  "manifest_url": "https://snapshots.elastic.co/9.2.0-8077f0d2/manifest-9.2.0-SNAPSHOT.json",
  "summary_url": "https://snapshots.elastic.co/9.2.0-8077f0d2/summary-9.2.0-SNAPSHOT.html",
  "core_version": "9.2.0",
  "stack_build_id": "9.2.0-8077f0d2-SNAPSHOT"
}

testing/integration/testdata/.upgrade-test-agent-versions.yml

{
  "UpgradeToVersion": "9.2.0",
  "CurrentMajors": 1,
  "PreviousMajors": 1,
  "PreviousMinors": 2,
  "SnapshotBranches": [
    "9.1",
    "9.0",
    "8.19",
    "7.17"
  ]
}

---

Why is it important?

To be honest this is not that important, but it helps to maintain descriptions of such PRs properly formatted

Checklist

• I have read and understood the pull request guidelines of this project.
• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool
• I have added an integration test or an E2E test

Disruptive User Impact

None, this affects just the automated PR descriptions

How to test this PR locally

This is a CI-only change

Related issues

N/A

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This is an automatic backport of pull request #9376 done by Mergify.