Update submodule: `protobuf` to v26.1

## Description
Let's update to Protocol Buffers to [`v26.1`](https://github.com/protocolbuffers/protobuf/releases/tag/v26.1) from `v26.0`.

## Version or commit-id
212b8238741f6dbda8358abf931a65022f1a7a6f

## Additional context
Previous update:
 * https://github.com/google/mozc/issues/871
