Change exclude-newer type into optional string (#17121)

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## Summary

fix: #17103 

## Test Plan

The following settings will be enabled for the schema.

```toml
[tool.uv]
exclude-newer = "P7D"
```

crates/uv-resolver/src/exclude_newer.rs

@@ -635,7 +635,6 @@ impl schemars::JsonSchema for ExcludeNewerValue {
     fn json_schema(_generator: &mut schemars::generate::SchemaGenerator) -> schemars::Schema {
         schemars::json_schema!({
             "type": "string",
-            "pattern": r"^\d{4}-\d{2}-\d{2}(T\d{2}:\d{2}:\d{2}(Z|[+-]\d{2}:\d{2}))?$",
             "description": "Exclude distributions uploaded after the given timestamp.\n\nAccepts both RFC 3339 timestamps (e.g., `2006-12-02T02:07:43Z`) and local dates in the same format (e.g., `2006-12-02`), as well as relative durations (e.g., `1 week`, `30 days`, `6 months`). Relative durations are resolved to a timestamp at lock time.",
         })
     }

uv.schema.json

@@ -893,8 +893,7 @@
     },
     "ExcludeNewerTimestamp": {
       "description": "Exclude distributions uploaded after the given timestamp.\n\nAccepts both RFC 3339 timestamps (e.g., `2006-12-02T02:07:43Z`) and local dates in the same format (e.g., `2006-12-02`), as well as relative durations (e.g., `1 week`, `30 days`, `6 months`). Relative durations are resolved to a timestamp at lock time.",
-      "type": "string",
-      "pattern": "^\\d{4}-\\d{2}-\\d{2}(T\\d{2}:\\d{2}:\\d{2}(Z|[+-]\\d{2}:\\d{2}))?$"
+      "type": "string"
     },
     "ExtraBuildDependencies": {
       "type": "object",