fix: slider disabled and readonly cursor styles

[SethDonohue]

Description

Closes #4222

Note: There is no .slider class on this component's template, so the css .slider selector was not targeting anything. The disabled cursor still works on the labels and the forcedColors cursor style can be removed with this change as well.

Before:

After:

High Contrast:

Issue type checklist

• Chore: A change that does not impact distributed packages.
• Bug fix: A change that fixes an issue, link to the issue above.
• New feature: A change that adds functionality.

Is this a breaking change?

• This change causes current functionality to break.

Adding or modifying component(s) in @microsoft/fast-components checklist

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

Process & policy checklist

• 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.

[EisenbergEffect]

We recently did some changes in other components to move away from being based on host classes and instead leverage attribute selectors, as that's less invasive and less likely to conflict with frameworks that may be interacting with our components. Would it be possible to take that same approach in this case as well?

[SethDonohue]

We recently did some changes in other components to move away from being based on host classes and instead leverage attribute selectors, as that's less invasive and less likely to conflict with frameworks that may be interacting with our components. Would it be possible to take that same approach in this case as well?

Updated to use attributes instead of classes. Will do so on anything in the future!

[marjonlynch]

smoked, looks good