[Load Testing] Fix broken reference doc links

[mccoyp]

Description

The URL paths for REST API reference docs recently changed, resulting in broken links.

This updates the broken links to use the new /apptesting/loadtest format.

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[Copilot]

Pull Request Overview

This PR updates documentation reference URLs in the README from legacy docs.microsoft.com domains to the current learn.microsoft.com domains, specifically updating Azure Load Testing API reference links to point to the new /apptesting/loadtest/ path structure.

• Updated API reference documentation URL from /rest/api/loadtesting/ to /rest/api/apptesting/loadtest/
• Updated data-plane URI documentation URL to use the new path structure
• Migrated URLs from docs.microsoft.com to learn.microsoft.com domain

Comments suppressed due to low confidence (1)

sdk/loadtesting/azure-developer-loadtesting/README.md:290

• This URL reference still uses the legacy docs.microsoft.com domain. For consistency with the other updates in this PR, consider updating it to learn.microsoft.com: https://learn.microsoft.com/azure/cognitive-services/authentication?tabs=powershell#authenticate-with-an-authentication-token

[authenticate_with_token]: https://docs.microsoft.com/azure/cognitive-services/authentication?tabs=powershell#authenticate-with-an-authentication-token

[mccoyp]

/check-enforcer override

The Analyze failure is from an unrelated Mypy issue.