Abuse report: Fix a11y issues

What does this MR do and why?

Abuse report: Fix a11y issues

• Fixes labels not being correctly defined
• Fixes wrong readonly styles (https://gitlab-org.gitlab.io/gitlab-ui/?path=/story/base-form-form-input--readonly)
• Makes category field visible, readonly (so it's clear what a user reports and has selected the step before)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Install https://chromewebstore.google.com/detail/axe-devtools-web-accessib/lhdoppojpmngadmnindnejefpokejbdd
2. Go to an issue
3. On the top right, click on the kebab menu
4. Choose «Report abuse»
5. Run the axe test
6. Click on «Next»
7. Run the axe test (http://gdk.test:3000/-/abuse_reports/add_category)

Related to #512691 (closed), !181288 (diffs, comment 2358026444)