Remove use of artifacts-api.elastic.co and its client

[rdner]

This includes removal of
TestStandaloneDowngradeToSpecificSnapshotBuild integration test.

Discussed with @pchila and @cmacknz

1. Independent agent patch releases will be ready very soon.
2. We are likely to handle downgrades in a different way than “upgrade to lower version” in the actual product.
3. This test fails our integration tests sue to instability of artifacts-api.elastic.co and that's the only code that is still using this API.

Closes #4347

[elasticmachine]

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

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[mergify]

This pull request does not have a backport label. Could you fix it @rdner? 🙏
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 8./d branch. /d is the digit

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

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b remove-TestStandaloneDowngradeToSpecificSnapshotBuild upstream/remove-TestStandaloneDowngradeToSpecificSnapshotBuild
git merge upstream/main
git push upstream remove-TestStandaloneDowngradeToSpecificSnapshotBuild

[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

[rdner]

@cmacknz are we still doing this?

[cmacknz]

I'm fine with it, this leaves some infrequently and not critical path code without a test, but eliminates the most frequent test failure.