Pin EC stack version to 8.x in CI

[1pkg]

Motivation/summary

This PR pins EC stack version in CI to the latest 8.x version until 9.x is ready.

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

All relevant CI pipelines should be using latest 8.x stack version.

Related issues

[mergify]

This pull request does not have a backport label. Could you fix it @1pkg? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• backport-8.x is the label to automatically backport to the 8.x branch.

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label.

[1pkg]

This is already addressed in a different branch.