Change template helper to use async partials

[louisjrdev]

Prerequisites

• I have added steps to test this contribution in the description below

If there's an existing issue for this PR then this fixes #11220

Description

Changes the templatehelper service so that when inserting a partial view in the template editor it uses the recommended approach of async partials.

Testing

To test this create a new document type with a template and create a content page for it. Now create a partial with some example content. Now return to the document type template and use the editor to insert a partial view and pick the one you just created. You should then save this and view your content page and see that it renders the partial content correctly.

[umbrabot]

Hi there @louisjrdev, thank you for this contribution! 👍

While we wait for one of the Core Collaborators team to have a look at your work, we wanted to let you know about that we have a checklist for some of the things we will consider during review:

• It's clear what problem this is solving, there's a connected issue or a description of what the changes do and how to test them
• The automated tests all pass (see "Checks" tab on this PR)
• The level of security for this contribution is the same or improved
• The level of performance for this contribution is the same or improved
• Avoids creating breaking changes; note that behavioral changes might also be perceived as breaking
• If this is a new feature, Umbraco HQ provided guidance on the implementation beforehand
• The contribution looks original and the contributor is presumably allowed to share it

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[bergmania]

Thanks.. This is an oversight, and should definitely be async :)

[bergmania]

Cherry picked for v9.0.1

[umbrabot]

Hi there @louisjrdev,

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