Make value_hint optional to allow constant positional arguments (#172)

<!-- Provide a brief summary of your changes -->

## Motivation and Context
<!-- Why is this change needed? What problem does it solve? -->

This is one attempt to resolve
https://github.com/modelcontextprotocol/registry/issues/169.

Instead of adding a new "constant" argument type, I expanded the
existing positional one and just dropped the `value_hint` requirement. I
am not sure if this has VS Code impact @sandy081, @connor4312.

Also, removed unneeded release_date values from the samples.

## How Has This Been Tested?
<!-- Have you tested this in a real application? Which scenarios were
tested? -->

I tested the current server.json samples in the repo against the new
(less strict) schema. Of course they passed (aside from an unrelated bug
in my previous NuGet sample 🤦).

## Breaking Changes
<!-- Will users need to update their code or configurations? -->

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [x] Documentation update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [ ] New and existing tests pass locally
- [ ] I have added appropriate error handling
- [x] I have added or updated documentation as needed

## Additional context
<!-- Add any other context, implementation notes, or design decisions
-->

docs/server-registry-api/openapi.yaml

@@ -247,7 +247,6 @@ components:
         - type: object
           required:
             - type
-            - value_hint
           properties:
             type:
               type: string
@@ -261,6 +260,11 @@ components:
               type: boolean
               description: Whether the argument can be repeated multiple times in the command line.
               default: false
+          anyOf:
+            - required:
+                - value
+            - required:
+                - value_hint
 
     NamedArgument:
       description: A command-line `--flag={value}`.