Follow-up to #27.
This PR adds a migration path for `Server::Transports::StdioTransport`. e.g.,
```console
$ cat /tmp/example.rb
require "mcp/transports/stdio"
MCP::Transports::StdioTransport
```
This change displays the following warning instead of raising an error for such cases, allowing execution to proceed:
```console
$ bundle exec ruby /tmp/example.rb
/tmp/example.rb:1: warning: Use `require "mcp/server/transports/stdio_transport"` instead of `require "mcp/transports/stdio"`.
Also use `MCP::Server::Transports::StdioTransport` instead of `MCP::Transports::StdioTransport`.
This API is deprecated and will be removed in a future release.
```
Currently (v0.1.0), it results in an error without any warning.
This MCP gem is not yet close to version 1.0, but introducing breaking changes to a published interface is undesirable.
This compatibility code should be removed at some point after a few releases.
Closes#50.
