docs: Update the text for experiment string
Description
Updates the warning text for experimental features. The new warning is shorter and more professional, simply stating that experimental features "are not ready for production use" and "might be unstable or removed at any time."
- "not ready for production use" comes directly from the development stages policy for experimental features
- "unstable or removed at any time" maps to two specific policy statements: "Might be unstable" and "Could be removed at any time."
- Removed "Use experimental features at your own risk" because the preceding sentences already communicate the risk clearly, and this phrasing is unnecessarily blunt for GitLab's voice