Stop supporting optional `tag:yaml.org,2002:bool` to ease adoption for GitHub Actions Workflow templating

**What steps did you take:**
Using `on: ` in a yaml file

**What happened:**
The `on: ` was replaced with `true: `

**What did you expect:**
As `on: ` is a valid field it should not have been converted to true

**Anything else you would like to add:**
This is very strange and impacts the ability to create templates for github actions.

**Environment:**

- `0.33.0`

Start by reproducing the reported `on:` to `true:` conversion with ytt 0.33.0 and compare it with current behavior. No source file or test is identified in the issue; done means GitHub Actions-style `on:` fields remain unchanged while other boolean handling continues to work.

Written by the indexing model from the issue text.