It was changed back in 4.94 as far as I remember, as case sensitivity increases the number of options making it more secure. I reommend you do not change this, but if you want to, you have to edit the library/ui/webcontrols/captchacontrol.vb and revert the code from (key changes in bold):

Public Function Validate(ByVal userData As String) As Boolean
            If String.Compare(userData, Me._CaptchaText, False, CultureInfo.InvariantCulture) = 0 Then
                _IsValid = True
            Else
                _IsValid = False
            End If
            RaiseEvent UserValidated(Me, New ServerValidateEventArgs(_CaptchaText, _IsValid))
        End Function

to:

Public Function Validate(ByVal userData As String) As Boolean
            If String.Compare(userData, Me._CaptchaText, True) = 0 Then
                _IsValid = True
            Else
                _IsValid = False
            End If
            RaiseEvent UserValidated(Me, New ServerValidateEventArgs(_CaptchaText, _IsValid))
        End Function

I will leave it be as I do not want to have to deal with this option everytime we upgrade.   However, this should be a configurable item like minimum password length, etc. is.