TaskType.LAMBDA should be removed — LAMBDA task type is deprecated

## Summary

`TaskType::LAMBDA` is present in the SDK's task-type enum (`lib/conductor/workflow/task_type.rb`) but the LAMBDA task type is **deprecated** in Conductor. New workflows should use `INLINE` instead. LAMBDA should be removed from the SDK's task-type list rather than built out.

## Current state

```ruby
Conductor::Workflow::TaskType::LAMBDA # => "LAMBDA" — still present
```

There is no `LambdaTask` builder class (the absence is intentional).

## Requested fix

Remove the `LAMBDA` constant from `TaskType` and drop it from `ALL_TYPES` in `lib/conductor/workflow/task_type.rb`.

## Note on scope (why this is safe here but not everywhere)

The Conductor **server still supports and emits LAMBDA** — it is deprecated, not removed. This SDK's `TaskType` is a builder/validation convenience (a string constant + an `ALL_TYPES` allow-list); it is **not** used to strictly deserialize workflow definitions into a typed enum. Removing the constant therefore only steers users away from building *new* LAMBDA tasks and does not break reading existing workflows that contain them.

This mirrors the Python SDK, where the same builder-enum removal was done in conductor-oss/python-sdk#427. The strongly-typed SDKs (Java, C#) are intentionally **not** getting this change, since removing a deprecated value from a deserialization enum while the server still emits it would break reading legacy workflow defs.

Start with lib/conductor/workflow/task_type.rb and inspect the TaskType constants and ALL_TYPES allow-list. Remove the deprecated LAMBDA entries while leaving the existing task-type behavior intact; done means LAMBDA is no longer offered for building new tasks and existing workflow definitions remain readable.

Written by the indexing model from the issue text.