testing: Fix Docker deprecation warnings

[mauri870]

Proposed commit message

I stumbled upon some warnings when trying to start the integration testing environment. The logs don't cause any harm but they pollute the output of the tool. Warnings include:

WARN[0000] /home/mauri870/git/elastic/beats/testing/environments/snapshot.yml: the attribute `version` is obsolete, it will be ignored, please remove it to avo
id potential confusion
WARN[0000] /home/mauri870/git/elastic/beats/testing/environments/local.yml: the attribute `version` is obsolete, it will be ignored, please remove it to avoid
potential confusion
WARN: MaintainerDeprecated: Maintainer instruction is deprecated in favor of using label (line 4)

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.

How to test this PR locally

• cd testing/environments; make start ENV=snapshot.yml

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @mauri870? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

NOTE: backport-v8.x has been added to help with the transition to the new branch 8.x.

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.