[EuiFieldText] Prevent onChange event on disabled inputs

[nickofthyme]

Summary

This alters the behavior of the EuiFieldText to prioritize disabled styling over readonly, the same as native HTML inputs.

Additionally, the disabled attribute does not prevent onChange events from firing. To counteract this behavior, we default the readonly attribute to the value of disabled unless it is explicitly set. With readonly set to true, the onChange event is prevented. Alternatively, we could conditionally call the onChange handler manually.

These changes were tested in kibana ftr in elastic/kibana#206372 with no failing tests.

Closes #8239

QA

General checklist

• Release checklist
  • A changelog entry exists and is marked appropriately.

[nickofthyme]

Looking for feedback at this point, no rush to merge.

[nickofthyme]

@tkajtoch if you don't like syncing the readonly attribute with the value of disabled, I could change this to a functional prevention such as to only call the onChange when disabled is false. Lmk

[tkajtoch]

I'm happy with this change. Could you please add a changelog item and a JSDoc comment to readOnly prop to let users know it defaults to the value of disabled now?

[tkajtoch]

LGTM! I'll merge it after today's EUI release

[kibanamachine]

Preview staging links for this PR:

• Docs site: https://eui.elastic.co/pr_8271/
• Storybook: https://eui.elastic.co/pr_8271/storybook
• New docs: https://eui.elastic.co/pr_8271/new-docs

[elasticmachine]

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