fix(GlModal): Update gl-modal to design guidelines

• Integration MR: gitlab!51807 (closed)

What does this MR do?

Based on the modal audit issue, this updates gl-modal to the styles outlined in Figma.

List of updates:

• Content borders changed to $gray-200
• Footer padding changed to 1rem
• Close button updated to gl-button
• The last <p> element is given a margin-bottom of 0. This made the content spacing unintentionally larger.
• Content font-size changed to 14px
• Line height for title and body changed to 20px
• Footer background changed to #fafafa
• Border of modal-content changed to transparent. Original guidelines say $gray-200, but this washes the edges out. I propose we create more solid edges by using a transparent border. This will also be more friendly with a dark theme since we don't have to reset this.
• Box shadow added to the modal-content
• The "Lightbox" background opacity was too light, changed to .48.
• Header title font-size changed to 16px and font-weight to 700.

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
  • CustomersDot: mr_url
  • Status Page: mr_url
• 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
• Security reports checked/validated by a reviewer from the AppSec team