docs: Clean up requirements, add Code Suggestions

Description

Updates README.md to condense down the requirements section, and clearly state the different requirements for Code Suggestions.

Related Issues

• Related to [VS Code] Code Suggestions: Minimal GitLab vers... (#1221 - closed)
• Related to Convert code suggestions version check to the s... (#1241 - closed)
• Related to Prep for Code Suggestions Service Start Date (gitlab-org/editor-extensions&41 - closed)
• Related to Minimum GitLab Version is required to use Code ... (&12397 - closed)
• Related to Updating for Duo Pro release (gitlab!143699 - merged) (updates the gitlab repo)

How has this been tested?

Screenshots (if appropriate)

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

Closes #1221 (closed)