minor #38187 [Form] Change 'cache_key' to AbstractRendererEngine::CACHE_KEY_VAR in...

This PR was merged into the 3.4 branch.

Discussion
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[Form] Change 'cache_key' to AbstractRendererEngine::CACHE_KEY_VAR in BaseType

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| License       | MIT

The purpose of this change is to find all usages of AbstractRendererEngine::CACHE_KEY_VAR. Currently, if you search for AbstractRendererEngine::CACHE_KEY_VAR you will see only access to it, i.e. (`$view->vars[AbstractRendererEngine::CACHE_KEY_VAR]`), but you can't find it in write level. With this pull request you can see where is was used for write.

Commits
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4fcb41c3ba Change 'cache_key' to AbstractRendererEngine::CACHE_KEY_VAR