Disable old example tooling

[timotheeguerin]

Part of #4831.

RFC: https://github.com/Azure/azure-rest-api-specs/blob/rfc/unified-examples-format/documentation/rfc/unified-examples-format.md (§9 Phase 3)

Disable the old Swagger-based example tooling once every service is migrated (#4839) and all consumers are on the new format (TCGC + consumers #4836).

Scope

• Turn off the legacy x-ms-examples validation/generation paths (e.g. OAV-based example handling superseded by examples-validate examples.yaml JSON Schema + examples-validate #4832 and examples-scaffold Auxiliary tooling: examples-scaffold + examples-diff #4837).
• Stop emitting the transitional x-ms-examples JSON from typespec-autorest (x-id emission: typespec-autorest + transitional JSON #4835); x-id becomes the sole linkage.
• New examples are authored only in the new format; CI enforces it.

Out of scope

• Deleting the legacy x-ms-examples JSON files (tracked separately).

Depends on #4839 (all services migrated) and #4836 (consumers on new format).