message_edit: Use disabled instead of readonly to disable controls.

[jai2201]

Currently when a user does not have the permission to edit the topic/content
of a message, the edit UI/view source UI correctly shows a greyed
out topic/message-content input field, however these fields incorrectly have a
click behavior, so to fix this we now would want to use disabled prop instead
of readonly attribute as readonly controls can still function and are still
focusable whereas disabled controls can not receive focus and are unclickable.

Fixes #22565

Fixes: 22565.

Screenshots and screen captures:

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[jai2201]

@timabbott , I looked at the other places where readonly was being used in this file, and it looked like they too needed to be changed to disabled , So I tested the PR changing them all to disabled and it looked to work prefect!
(with readonly attribute the message content field was still focusable which is not the case with disabled so the change looked correct to me;)

Added the reason in commit message;

[timabbott]

This is great, merged, thanks @jai2201!