Document supported MCP protocol versions

What does this MR do and why?

Problem

The mcp server .md file only listed 2025-03-26 and 2025-06-18 as supported protocol versions, but the server also accepts 2025-11-25 and 2026-07-28. There was also no documented deprecation policy.

Changes

• Adds a "Supported MCP protocol versions" section with a table covering all four versions:
  • 2025-11-25 — supported, and the version the server falls back to when a client requests something newer
  • 2025-06-18 and 2025-03-26 — still supported
  • 2026-07-28 — accepted only in initialize; server responds with 2025-11-25 because the stateless features aren't implemented yet
• Adds a history entry noting support for 2025-11-25 was added in GitLab 18.7
• Documents GitLab's deprecation policy:
  • A protocol version stays supported after the MCP spec deprecates it
  • Removing a version is a breaking change
  • Removals are announced on the deprecations and removals page first

Dependency

The documented 2026-07-28 behavior depends on the work in !253324 (merged). This MR should merge after that one, or at the same time.

Closes the "Document which protocol versions are supported and define a deprecation policy for older versions" acceptance criterion.

References

• Related issue: #611372 (closed)
• Follow-up issue for the stateless revision: #627825
• Depends on: !253324 (merged)

How to set up and validate locally

1. Check out the branch.
2. Open the file doc/user/model_context_protocol/mcp_server.md and confirm the new "Supported MCP protocol versions" section appears before "Related topics".
3. Run vale --minAlertLevel warning doc/user/model_context_protocol/mcp_server.md and confirm no alerts. Vale reported no alerts when run locally.

MR acceptance checklist

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