fix(GlDatepicker): fix x-axis padding

Closes #776 (closed)

What does this MR do?

Changes the datepicker x-axis padding from 0.75rem (12px) to 0.5rem (8px) to align with the Figma spec (https://www.figma.com/file/qEddyqCrI7kPSBjGmwkZzQ/Pajamas-UI-Kit?node-id=4592%3A7453).

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• 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 in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • Customers Portal: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.