FC-0001: rename toggle_warnings to toggle_warning

[dyudyunov]

Rename toggle_warnings to toggle_warning for consistency with setting_warning.

Relates to: openedx/code-annotations#80

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[dyudyunov]

@robrap please review

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[edx-pipeline-bot]

EdX Release Notice: This PR has been deployed to the staging environment in preparation for a release to production.

[edx-pipeline-bot]

EdX Release Notice: This PR has been deployed to the production environment.