[pixman] Correct invalid license string in vcpkg.json (MIT instead of MIT AND BSD-2-Clause)

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### Is your feature request related to a problem? Please describe.

The port for `pixman` (`ports/pixman/vcpkg.json`) currently declares its license as `"MIT AND BSD-2-Clause"`. However, the library `pixman` is licensed exclusively under the MIT license. There is no BSD-2-Clause component in the project.

### Proposed solution

Change the `"license"` field in `ports/pixman/vcpkg.json` from:
```json
"license": "MIT AND BSD-2-Clause"
```
to:
```json
"license": "MIT"
```

### Describe alternatives you've considered

_No response_

### Additional context

The license can be found directly in the official `pixman` repository: https://gitlab.freedesktop.org/pixman/pixman/-/blob/master/COPYING

This fix ensures that automated legal and compliance checking tools that use vcpkg manifests do not report incorrect dependencies related to the BSD license.

Open ports/pixman/vcpkg.json and inspect the current license field; the issue identifies the exact value to correct. Change it to MIT and validate that the manifest remains valid, with the completed change reflected in the pixman port metadata.

Written by the indexing model from the issue text.