feat: upgraded gitlab-ui to 68.7.0

Description

The MR upgrades @gitlab/ui to v.68.7.0 to introduce the copy-code button

In the same process, the MR also updates some styling of the Duo Chat messages

How has this been tested?

• running the extension in VSCode
• running the extension in Web IDE

Screenshots (if appropriate):

Styling updates:

Before	After

Web IDE with the default theme after:

The new copy-code button (in the latest version of gitlab-ui):

VSCode	Web IDE
copy-code-vscode-after	copy-code-webide-after

Types of changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap