fix: samples/server/petstore/python-fastapi/requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-PYTHONMULTIPART-16078395

Author: snyk-bot

samples/server/petstore/python-fastapi/requirements.txt

@@ -22,7 +22,7 @@ orjson==3.9.15
 promise==2.3
 pydantic>=2
 python-dotenv==0.17.1
-python-multipart==0.0.18
+python-multipart==0.0.26
 PyYAML>=5.4.1,<6.1.0
 requests==2.32.4
 Rx==1.6.1