feat(rux-textarea) add readonly attribute

[kiley-mitti]

Brief Description

Added readonly attribute to rux-textarea in a way that mimics rux-input

JIRA Link

ASTRO-6600

Related Issue

General Notes

Motivation and Context

I've run into some dev instances in the TTC/GRM apps where it would have been nice to have readonly available.

Issues and Limitations

Types of changes

• [ ] Bug fix
• [x] New feature
• [ ] Breaking change

Checklist

• [ ] This PR adds or removes a Storybook story.
• [ ] I have added tests to cover my changes.
• [x] Regressions are passing and/or failures are documented
• [x] Changes have been checked in evergreen browsers

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: bb7113415f63c3d107afc998f344a2a9873db7f7

The changes in this PR will be included in the next version bump.

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[netlify[bot]]

✅ Deploy Preview for astro-stencil canceled.

Name	Link
🔨 Latest commit	bb7113415f63c3d107afc998f344a2a9873db7f7
🔍 Latest deploy log	https://app.netlify.com/sites/astro-stencil/deploys/64d2ac798901770008cfcbde

[netlify[bot]]

✅ Deploy Preview for astro-preview canceled.

Name	Link
🔨 Latest commit	bb7113415f63c3d107afc998f344a2a9873db7f7
🔍 Latest deploy log	https://app.netlify.com/sites/astro-preview/deploys/64d2ac79832c1f00086b5305