[8.18] (backport #8761) ci: always include 8.19 LTS release branch in snapshots of test versions

[mergify]

What does this PR do?

This PR ensures the inclusion of the 8.19 LTS branch in the list of snapshot branches used for upgrade testing in CI. Previously, only a limited number of recent release branches (e.g., 9.x, 7.17) were included. However, since 8.19 is designated as an LTS branch and will be supported for an extended period, this change adds it explicitly - similar to the existing logic for 7.17.

The implementation includes:

• Dynamically appending 8.19 to the list of snapshot branches if the current agent version is greater than 8.19.
• Updating testdata/.upgrade-test-agent-versions.yml to reflect the latest 7.17 snapshot version (7.17.30-SNAPSHOT).

Why is it important?

This change ensures that upgrade tests against the 8.19 LTS branch are consistently included in CI pipelines, especially when building agent versions from 9.x onward. It guarantees test coverage against supported upgrade paths and aligns with the long-term support strategy.

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 change only affects internal test version selection logic for CI environments.

How to test this PR locally

mage integration:updateVersions

Related issues

N/A

---

This is an automatic backport of pull request #8761 done by Mergify.

[mergify]

Cherry-pick of e629b9e has failed:

On branch mergify/bp/8.18/pr-8761
Your branch is up to date with 'origin/8.18'.

You are currently cherry-picking commit e629b9ee5.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   magefile.go
	modified:   pkg/testing/tools/git/git.go
	modified:   pkg/testing/tools/git/git_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   testing/integration/testdata/.upgrade-test-agent-versions.yml

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarQube