Support multiple schema of server.json

[pree-dew]

Details mentioned here https://github.com/modelcontextprotocol/registry/pull/488#issuecomment-3298998139

At any point of time, registry should support multiple server.json versions, so that it's not mandatory to update the version, allows flexibility to update when user wants to update.

[tadasant]

The rough idea of what I was thinking here is:

• Publishing process should have an explicit list of supported versions, with logic branches to handle each possibility
• Consumption process should as well

I think we can generally be more strict on publishing than consumption -- requiring an update to push an update feels significantly less heavy-handed than requiring all downstream sub-registry and MCP client consumers to update their implementations.

But maybe there's an argument to keeping them in sync? I think we could build a better experience by decoupling them, but open to other opinions.

[tadasant]

See https://discord.com/channels/1358869848138059966/1369487942862504016/1417915364289020045 for discussion on server.json immutability