Update rich text editor dependency and associated changes

[alunturner]

Updates the RTE repo and makes the required associated changes:

• the RTE repo now exposes a function specifically for handling at room mentions, so we use this
• change of types for the calls to functions relating to mentions (from object to map)
• these changes have been applied through the rich and plain modes of the rich text editor and required some changes to utils to accommodate
• the RTE repo now exposes the composer content formatted as a matrix spec message, so we use this in the message content
• updates unit tests as required

Checklist

• Tests written for new code (and old code if feasible)
• Linter and other CI checks pass
• Sign-off given on the changes (see CONTRIBUTING.md)

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Here's what your changelog entry will look like:

✨ Features

• Update rich text editor dependency and associated changes (#11098). Contributed by @alunturner.

[alunturner]

This is the change that ensures our message content, when sent, is correctly formatted according to the matrix spec as opposed to being exactly the markup used in the html inside the composer.

[alunturner]

This was an error I picked up working in the area, the content of the composer needs to be run through amendInnerHtml before being stored in the content state.