When using ember-template-tag (.gjs), templates are much more interlinked with the JS file they're defined in. Therefore, attempting to override their template with a 'non-strict-mode' template doesn't make sense, and will likely lead to problems.
This commit skips any such overrides, and introduces a clear console warning. In theme/plugin tests, an error will be thrown during app boot.
Going forward, we aim to provide alternative APIs to achieve the customizations people currently implement with template overrides. (e.g. https://github.com/discourse/discourse/pull/23110)
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