Spring Deprecate local variable support in SpEL's internal ExpressionState

As discussed in https://github.com/spring-projects/spring-framework/issues/23202#issuecomment-1884975107, the Spring Expression Language does not actually support local variables in expressions.

Consequently, we will deprecate all public APIs related to local variables in ExpressionState (enterScope() variants that accepts local variable data, setLocalVariable(), and lookupLocalVariable()) and stop invoking state.enterScope("index", index) in Projection and Selection.