Publicize: LinkedIn Custom Message and Title

#### Steps to reproduce the issue

1. Start on https://wordpress.com/block-editor/post and write a post.
2. Add a custom Publicize message to the post in the **Share this post** section under Jetpack.
3. Publish the post!

#### What I expected

I expected that the custom Publicize message will be an addition to the shared blog post.

#### What happened instead

Instead, the custom message will overwrite the post title on the LinkedIn shared post card.

Screenshot:

Post in LinkedIn - https://www.linkedin.com/feed/update/urn:li:activity:6570254554229809152/

First mentioned here #11752, but created a separate issue for tracking purposes.
cc @jeherve

Start by reproducing the issue from the WordPress block editor's Jetpack Share this post section, then inspect the Publicize flow for LinkedIn sharing. Done means the custom message appears as an addition while the LinkedIn shared-post card retains the original post title.

Written by the indexing model from the issue text.