Rename "Text alternative" field label to "Alternative text"

Problem/Motivation

The alternative text field in the CKEditor5 image dialog reads "Text alternative". It would be better if it read "Alternative text". This would be consistent with the CKEditor media dialog as well as with how this content is normally referred to in the accessibility community.

Steps to reproduce

Steps:
1. Go to simplytest.me
2. Type core in the field
3. Select Drupal core, latest version
4. Click the button to create the site
5. Log in as admin
6. If the management menu is not displayed, click Manage
7. Click content
8. Click add content
9. Click Basic page
10. Click in the body field
11. Click the image icon in the WYSIWYG toolbar

Expected result: alternative text field is labeled "Alternative text"
Actual result: alternative text field is labeled "Text alternative"

Proposed resolution

Edit "Text alternative" to "Alternative text"

Merge request link

Remaining tasks

User interface changes

Visible field label in image dialog changes from "Text alternative" to "Alternative text".

API changes

Data model changes

Release notes snippet