Mark the GraphQL API deprecated and list the migration guide

[g-despot]

Docs side of the GraphQL API deprecation announced on October 12, 2026: deprecated now, switched off on Weaviate Cloud on April 1, 2027, disabled by default in self-hosted Weaviate in a later minor.

• _includes/graphql-deprecation.mdx — one shared warning with the dates and a link to the guide, imported on the 8 api/graphql/ reference pages and on concepts/interface.md
• api/graphql/migration.mdx — now listed. New "What is changing and when", "Do you need to migrate" (find your GraphQL usage, test with GraphQL disabled) and "GraphQL features without an equivalent" sections
• sidebars.js — category renamed "GraphQL (deprecated)", gRPC moved above it, the guide listed first; the unlisted-banner swizzle is removed
• api/index.mdx, api/graphql/index.md, api/grpc.md — gRPC presented as the search API, GraphQL as deprecated; C# added to the client list
• DISABLE_GRAPHQL env-var row, Cloud FAQ and default settings, Go client page — dates and the Go v5 warning
• docusaurus.config.js — announcement bar swapped to the deprecation

Hold until October 12. Merge before the blog post weaviate/weaviate-io#3723 goes live, which links to the guide's anchors. TODO(ivan) comments mark what still depends on engineering: the self-hosted minor that flips the default, the release status of REST Search and Go v6, and the fate of GraphQL-only features.

[orca-security-eu]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   0   0   0	View in Orca
Passed	SAST	0   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca