Incorrect Prefix Mapping in context.json

I noticed that the current context file ([context.json](https://github.com/dandi/schema/blob/master/releases/0.6.9/context.json#L29C5-L29C28)) is generating an incorrect mapping for the Activity class. Specifically, it maps Activity to dandi:Activity, while it should instead reference the prov:Activity from the PROV ontology.

Interestingly, the prov prefix is already defined in the context, but it is not being utilized.

**Expected Behavior:**

The context should reference:

```YAML
"Activity": "prov:Activity"
```

**Why This Matters:**
Using the incorrect prefix implies that Activity is a custom DANDI-defined term rather than a standard term from the PROV ontology. This breaks semantic alignment with existing ontologies/schemas and hinders interoperability, which is critical for downstream applications and FAIR compliance.

**Suggested Fix:** Use the correct prefix.

`Note:` Since I am familiar with `PROV` ontology so I observe the issue but there might be issues with other classes/properties. It is advisable that we do check all classes/properties to ensure that they're using the correct prefix.

@yarikoptic @candleindark @kabilar