fix(checkbox, radio): update border colors to match MD2 spec (#27357)

Issue number: resolves #27170

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Updates the Material Design border colors for checkbox and radio to
`rgb($text-color-rgb, 0.60)` or `#666666`. This is based on the
[Material Design 2
kit](https://www.figma.com/community/file/778763161265841481/Material-2-Design-Kit).

The border color when disabled is also wrong, but changing this affects
the label so I have reverted this to do as part of FW-4037.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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Co-authored-by: ionitron <hi@ionicframework.com>
Co-authored-by: Liam DeBeasi <liamdebeasi@users.noreply.github.com>

core/src/components/checkbox/checkbox.md.vars.scss

@@ -38,7 +38,7 @@ $checkbox-md-icon-border-style:          solid !default;
 $checkbox-md-icon-border-radius:         2px !default;
 
 /// @prop - Border color of the checkbox icon when off
-$checkbox-md-icon-border-color-off:      rgba($text-color-rgb, .51) !default;
+$checkbox-md-icon-border-color-off:      rgb($text-color-rgb, 0.60) !default;
 
 /// @prop - Border color of the checkbox icon when on
 $checkbox-md-icon-border-color-on:       ion-color(primary, base) !default;