Update prev-minor testing environment to 8.0 on master (8.1)

[kvch]

What does this PR do?

This PR updates the prev-minor.yml environment to the latest 8.0.0 release on master. Master is at the moment 8.1, hence the previous minor is 8.0.0.

Why is it important?

Without this lots of module tests fail. Also, the name implies previous minor version. But in reality it was set to 7.15 which is not the previous minor.

Checklist

- [ ] 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.next.asciidoc or CHANGELOG-developer.next.asciidoc.

[botelastic]

This pull request doesn't have a Team:<team> label.

[mergify]

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

• backport-v./d./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[kvch]

Relevant tests have passed. Merging.

[elasticmachine]

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Build stats

• Start Time: 2022-01-26T14:35:37.627+0000

• Duration: 121 min 36 sec

• Commit: 1837add

Test stats 🧪

Test	Results
Failed	0
Passed	47950
Skipped	4285
Total	52235

💚 Flaky test report

Tests succeeded.

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