fix: unpin duckduckgo-search==6.4.1 (yanked from PyPI)

Version 6.4.1 is no longer distributed on PyPI (6.4.2 is the
closest available in the 6.4.x line). Four requirements.txt
files still pin the unavailable version, causing a hard
`pip install` failure on a clean checkout.

Widen to `>=6.4.2,<9` so the resolver picks the latest
compatible release (currently 8.1.1); the lower bound preserves
the original intent of pinning within the 6.4.x family, and the
upper bound matches the loosest existing pin elsewhere in the
repo (ai_competitor_intelligence_agent_team uses 7.2.1, the
ADK crash course uses `>=6.0.0`).

Affected projects:
- starter_ai_agents/ai_medical_imaging_agent
- rag_tutorials/rag_database_routing
- rag_tutorials/rag_agent_cohere
- advanced_ai_agents/multi_agent_apps/agent_teams/ai_teaching_agent_team

Fixes #63

Made-with: Cursor

rag_tutorials/rag_database_routing/requirements.txt

@@ -8,4 +8,4 @@ sentence-transformers>=2.2.2
 agno
 langchain-openai==0.2.14
 langgraph==0.2.53
-duckduckgo-search==6.4.1
+duckduckgo-search>=6.4.2,<9