Update name of heartbeat config for Cloud

[jlowin]

Thanks for contributing to Prefect!

Please describe your work and make sure your PR:

• adds new tests (if appropriate)
• updates CHANGELOG.md (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?

The config options in Cloud have been renamed to have positive/descriptive values, so disable_heartbeat has become heartbeat_enabled.

The change is backwards-compatible, so old versions of Core will continue to function.

This PR should not be merged until this name change has been released to Cloud users.

[codecov]

Codecov Report

Merging #2081 into master will increase coverage by 0.1%.
The diff coverage is 100%.

[jlowin]

@joshmeek this can be merged anytime