GHA: add yamlcheck

Uses `yamllint`.

Also:
- fix warning for existing YAML files:
```
[truthy] truthy value should be one of [false, true]
```

Closes #

GHA: add yamlcheck

Uses `yamllint`.

Also:
- fix warning for existing YAML files:
```
[truthy] truthy value should be one of [false, true]
```

Closes #14650

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