fix(foundation): remove readonly from Slider

[olaf-k]

📖 Description

This PR removes the readonly state for the Slider component in an effort to keep it close to the input range's behavior.
The specs for the latter can be found here (see the 'Bookkeeping details' part after the examples).

See PR #6438 for a similar update to the radio button.

👩‍💻 Reviewer Notes

As it is a removal, the change is pretty straightforward. Code/tests/doc should be covered.
slider-label should not be affected as far as I can tell.

✅ Checklist

General

• I have included a change request file using $ yarn change
• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.

Component-specific

• I have added a new component
• I have modified an existing component
• I have updated the definition file
• I have updated the configuration file

[olaf-k]

@microsoft-github-policy-service agree company="Vonage"

[EisenbergEffect]

This PR seems very similar in nature to the one we have for radio groups that makes the same set of changes. Like that one, I want to make sure we don't have a bug in the key handler. Shouldn't it be checking for disabled state like the mouse handler or does the platform handle that automatically?

[olaf-k]

hey team! do you think it'd be possible to push this forward to complete our great ReadOnly Purge? 😁