Retain commit summary/description focus while committing

[niik]

Closes #17283

Description

Switches our our method for preventing input into the commit summary, description, and co-authors field while committing to using the readonly attribute rather than disabled. From MDN:

The difference between disabled and readonly is that read-only controls can still function and are still focusable, whereas disabled controls can not receive focus and are not submitted with the form and generally do not function as controls until they are enabled.

I ran this past our a11y folks and they had no objections to this approach.

Release notes

Notes: [Improved] Commit text inputs retain focus while committing

[sergiou87]

Code looks good and works as expected!! I did some other testing looking for regressions but couldn't find any 🤞