Fix #11929 custom title boost value for Elasticsearch

[krukas]

Solves: #11929

[squash-labs]

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Test this branch here: https://krukasmain-z29di.squash.io

[gasman]

Thanks for picking this up. I don't think this is the right solution, though - Wagtail doesn't have any established rule about whether your apps are placed above or below Wagtail's apps in INSTALLED_APPS, so a change that fixes the issue for your project is just as likely to break someone else's.