WB-1672: Form fields: Update border color to offBlack50

[jandrade]

Summary:

For a11y reasons, we are changing the border color from offBlack16 to offBlack50. This will solve a color contrast issue for non-text elements like textfields and dropdown openers.

This PR updates this color in TextField, MultiSelect and SingleSelect to match these changes.

TextField / LabeledTextField:

MultiSelect / SingleSelect:

Issue: WB-1672

Test plan:

Verify the border color of the Textfield, MultiSelect and SingleSelect in Storybook and chromatic, and ensure that the color looks as we expect.