fix: examples/snowflake/requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-PYMONGO-7172112

Author: snyk-bot

examples/snowflake/requirements.txt

@@ -1,4 +1,5 @@
 rgsync
 snowflake-sqlalchemy
 
-cryptography>=3.2 # not directly required, pinned by Snyk to avoid a vulnerability
+cryptography>=3.2 # not directly required, pinned by Snyk to avoid a vulnerability
+pymongo>=4.6.3 # not directly required, pinned by Snyk to avoid a vulnerability