Fix ability for unprivileged Windows to upgrade

[blakerouse]

What does this PR do?

Adds the SeCreateSymbolicLinkPrivilege to the created elastic-agent-user when installing in unprivileged mode.

Why is it important?

This privilege is required on the `elastic-agent-user. With out the privilege upgrade fails to update symlinks between the current and previous versions of Elastic Agent.

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 integration tests
• [ ] I have added an entry in ./changelog/fragments using the changelog tool
• I have added an integration test or an E2E test

Related issues

• Closes Unable to upgrade in unprivileged mode on Windows #4646

[elasticmachine]

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

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube

[blakerouse]

I was able to confirm that this does fix the issue. I have applied this change to #4642 and ran a single test of TestStandaloneUpgradeWithGPGFallback on Windows. That test specifically installs the build version and then upgrades to the snapshot version. That ensures that the user is created with this new permission and that an upgrade can succeed, the snapshot version doesn't need this change to upgrade to because the user is already created.

The tests that install with the snapshot first and then upgrade do need this change. That is why I separated this change from #4642.