chore: update CLI analytics to use heroku credential manager

[erika-wallace]

Summary

This PR updates CLI analytics to use the credential manager for retrieving a user's authentication token.

• Updates BackboardHerokulyticsClient to use APIClient from @heroku-cli/command, which handles credential resolution (both HEROKU_API_KEY and stored credentials) through the shared credential manager
• Removes unused user and usingHerokuAPIKey getters

Type of Change

Breaking Changes (major semver update)

• Add a ! after your change type to denote a change that breaks current behavior

Feature Additions (minor semver update)

• feat: Introduces a new feature to the codebase

Patch Updates (patch semver update)

• fix: Bug fix
• deps: Dependency upgrade
• revert: Revert a previous commit
• chore: Change that does not affect production code
• refactor: Refactoring existing code without changing behavior
• test: Add/update/remove tests

Testing

Notes:

Steps:

1. Pull down this branch
2. heroku logout
3. npm install && npm run build
4. ./bin/run login
5. DEBUG=heroku:analytics ./bin/run whoami — in the debug output, confirm you see heroku:analytics Successfully sent Herokulytics data
6. ./bin/run logout
7. DEBUG=heroku:analytics ./bin/run whoami — in the debug output, confirm you see heroku:analytics Successfully sent Herokulytics data

Screenshots (if applicable)

Related Issues

GUS work item: W-20867247

[tlowrimore-heroku]

LGTM!