Style form_input/textarea readonly example

What does this MR do?

A tiny MVC for: #759 (closed)

In the text area example, the plaintext/readonly example shows the incorrect styles as it does not have disabled applied to it. To fix that, we add the correct property to the example ensuring the styles are correct.

The disabled styles for the regular form_input.vue are correct 🍾

Screenshots

Questions

1. In the design guidelines, I cannot find a style guide for just a plaintext property, nor does the regular form input have an example for this. Should we both add an example there and update the styles of the bootstrap-vue plaintext to mimic the disabled styles?

Does this MR meet the acceptance criteria?

Conformity

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