Core: Deprecate `QUnit.load()` and document migration path

[Krinkle]

[!NOTE] Migration guide has been published at: https://api.qunitjs.com/QUnit/load/

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Based on https://github.com/qunitjs/qunit/pull/1502 by @smcclure15. Closes https://github.com/qunitjs/qunit/issues/1084.