Fix deprecated Kibana APIs

[mrodm]

Follows #2233

This PR solves these two issues related to 9.0.0-SNAPSHOT:

• Fixed API to get the list of Elastic Agents available and the API to reassign one policy to an Elastic Agent.
• Fixed Agent Policy name to be used in Elastic Agents when testing with Kubernetes (kind).

It has been kept the backwards compatibility to use the previous calls or fields in older stacks (<9.x).

Related PRs:

• [Fleet] Remove deprecated APIs for agents endpoints kibana#198313

Author's checklist

• Ensure elastic-package stack up runs successfully.
• Run locally elastic-package test system -v with a local Elastic stack 9.0.0-SNAPSHOT
• Install package and try to edit and export dashboards
• Test packages in the integrations repository with a local Elastic stack 9.0.0-SNAPSHOT
  • This build could be taken as a reference: https://buildkite.com/elastic/integrations/builds/17827
• Test packages in the integrations repository, each package with the required Elastic stack version.
  • [DO NOT MERGE] Test fixes in APIs used by elastic-package integrations#11790
• Test packages with a Elastic Serverless project.
  • https://buildkite.com/elastic/elastic-package-test-serverless/builds/213

[mrodm]

test integrations

[elastic-vault-github-plugin-prod]

Created or updated PR in integrations repository to test this version. Check elastic/integrations#11789

[mrodm]

buildkite test serverless

[elastic-vault-github-plugin-prod]

Triggered serverless pipeline: https://buildkite.com/elastic/elastic-package-test-serverless/builds/213

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 8553856

cc @mrodm

[mrodm]

test integrations

[elastic-vault-github-plugin-prod]

Created or updated PR in integrations repository to test this version. Check elastic/integrations#11789

[kpollich]

Thanks for fixing this 🚀