Remove reference to graphql/alpha

[jlowin]

Thanks for contributing to Prefect!

Please describe your work and make sure your PR:

• adds new tests (if appropriate)
• add a changelog entry in the changes/ directory (if appropriate)
• updates docstrings for any new functions or function arguments, including docs/outline.toml for API reference docs (if appropriate)

Note that your PR will not be reviewed unless all three boxes are checked.

What does this PR change?

This PR removes an old reference to graphql/alpha endpoint in favor of graphql. Requests to graphql/alpha will still work.

Why is this PR important?

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@fce5eef). Click here to learn what that means.
The diff coverage is n/a.