Remove !important clauses from button disabled styles

What does this MR do?

The !important clauses in GitLab’s button [disabled] styles overwrite GitLab UI Button component’s [disabled] styles. This MR replaces the !important clauses with using a :not(.disabled) pseudo-selector where disabled styles are incorrectly overwritten.

Screenshots

I tested the following scenarios:

• A button with a variant.
• A button with an inverted variant.
• A button with a text-field style variant.
• A button with a btn-link variant.

Except for the btn-link variant, when disabled, all buttons look like:

A disabled btn-link button looks like this even when hovered:

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team