Add ENV variable ES_MULTILINGUAL_INDEX to default ENV variables

[M-arcus]

1. Why is this change necessary?

This ENV variable is required to use Advanced Search 2.0, see installation requirements at https://developer.shopware.com/docs/products/extensions/advanced-search/installation.html

2. What does this change do, exactly?

It adds ENV variable ES_MULTILINGUAL_INDEX to default ENV variables

3. Describe each step to reproduce the issue or behaviour.

Run fully licensed Shopware 6.5.6.1 with Shopware Commercial installed but no ES_MULTILINGUAL_INDEX set.
The config settings for Advanced Search do not appear in the Administration settings.

4. Please link to the relevant issues (if any).

5. Checklist

• I have rebased my changes to remove merge conflicts
• I have written or adjusted the documentation according to my changes

[lx-wnk]

Hey @M-arcus thank you for your contribution. Unfortunately, we have to reject it due to the existing feature flag. We try to offer only a standard setup with a minimum of predefined changes to ensure that it works with every project.

[M-arcus]

@jinnoflife I will add a feature flag and open a new PR