Merge app and request contexts into a single context (#5639)

This PR implements the proposal from #5639 to merge the app and request contexts into a single execution context. This simplifies the implementation by:

1. Creating a new ExecutionContext class that combines both app and request contexts
2. Using a single context variable _cv_execution instead of separate _cv_app and _cv_request
3. Maintaining backward compatibility by setting both old context vars when needed
4. Adding deprecation warnings for the old context methods
5. Updating all globals to use the new execution context

Key changes:

• Added new ExecutionContext class in ctx.py that handles both app and request contexts
• Updated globals.py to use the new execution context for all proxies
• Modified app.py to create execution contexts instead of separate app/request contexts
• Added deprecation warnings for has_app_context() and has_request_context()
• Simplified context management by removing the need to track multiple context stacks

The changes maintain backward compatibility while paving the way for a cleaner implementation in Flask 4.0. The new execution context:

• Has a single context variable instead of two
• Handles both app and request state in one place
• Maintains the same behavior for existing code
• Provides clearer error messages when contexts are missing

This change will make the codebase simpler and easier to understand, while also making it more efficient by reducing the number of context variables that need to be managed.

Closes #5639