Use design tokens in GlDatepicker - Button styles

What does this MR do?

I split the work for the GlDatepicker into multiple MRs. This one is actually replacing styles defined in the button.scss. I added screenshots to show the new values within the Datepicker. Furthermore it has impact on some styles on the buttons. I added screenshots for them as well.

Screenshots or screen recordings

Table of Screenshots of GlDatepicker

Figma	Screenshots
	Active & Focus:

Button Screenshots and Recordings

Screen_Recording_2024-07-04_at_14.09.19

Keep Datepicker open

Probably not needed within this MR, but still want to mention it:

If you need to keep the Datepicker open and click around in Dev Tools, you can go to the close callback and add the line this.show(); to immediately show the datepicker again. In the browser, it will not flicker.

Full code:

    closed() {
      /**
       * Emitted when the datepicker is hidden.
       */
      this.$emit('close');
      this.show();
    },

Integration merge requests

Not required

Does this MR meet the acceptance criteria?

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Conformity

• Code review guidelines.
• GitLab UI's contributing guidelines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened (see integration merge requests above).
• Added the ~"component:*" label(s) if applicable.

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
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Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.

Related to #2686