docs: update GitLab maintained versions documentation

Description

This change updates the documentation to clarify which versions of GitLab work with the Visual Studio Code extension. Instead of listing specific version numbers, it now explains that the extension officially supports only currently maintained GitLab versions (following GitLab's standard support policy). The update also reorganizes the requirements section to be clearer about what's needed for different features - like basic functionality, AI coding assistance, and security scanning - making it easier for users to understand what they need for the specific features they want to use.

Related Issues

Resolves #[issue_number]

How has this been tested?

• If src/browser or src/common has been modified, please consider interoperability with the Web IDE. See Running the Extension in WebIDE.
• Consider an end-to-end test for significant new features that aren't covered by integration tests.

Screenshots (if appropriate)

What CHANGELOG entry will this MR create?

• fix: Bug fix fixes - a user-facing issue in production - included in changelog
• feature: New feature - a user-facing change which adds functionality - included in changelog
• BREAKING CHANGE: (fix or feature that would cause existing functionality to change) - should bump major version, mentioned in the changelog
• None - other non-user-facing changes