chore: Simplify unsupported-version messages

Description

While I was out sick, @viktomas asked me to review some updates:

• Error messaging in !1469 (comment 1807260109)
• A new table in the README in !1487 (comment 1825161626)

I'm still cleaning up after missing a week of work, so here's my post-merge review of the text.

Related Issues

For the error messaging:

• Related to feat: detect unsupported GitLab version when ad... (!1469 - merged)
• Related to Notify user that the GitLab instance is not sup... (#824 - closed)

For the README change:

• Related to docs: mention code suggestions status bar item ... (!1487 - merged)

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