Warn user when a deprecated Data Value is being set

**Describe the problem/challenge you have**
#514 introduces the `@schema/deprecated` annotation.
However, when a data value that has been annotated as such, there's no indication to the user that they are doing so.

**Describe the solution you'd like**
When a data value is being set and its been marked as `@schema/deprecated`, report a warning (into stderr) that includes the notice

```yaml
#@data/values-schema
---
...
#@schema/deprecated "Set 'Version' with the semver version, instead."
Vers: "latest"
Version: "1.1.0"
```console
ytt: Warning: The data value "NoLongerUsed" has been deprecated: Set 'Version' with the semver version, instead.
```

**Anything else you would like to add:**
See also: https://hackmd.io/pODV3wzbT56MbQTxbQOOKQ#schemadeprecated

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