A6, A7, PIN_A6, PIN_A7 exist when there are not 8 analog pins

The standard variant always creates the defines PIN_A6 and PIN_A7 and the constants A6 and A7 even when there are not 8 analog inputs.

The variant eightanaloginputs exists to declare that the h/w as 8 analog inputs so why not use that in the standard variant to disable the extra two inputs when they don't exist.
It only requires a slight tweaking of the files.

i.e.
In eightanalog inputs pins_arduino.h

```
#define NUM_ANALOG_INPUTS 8
#include "../standard/pins_arduino.h"
```

And in the standard pins_arduino.h

```
// assume 6 analog inputs if not told 8 by another variant file including this file
#ifndef NUM_ANALOG_INPUTS
#define NUM_ANALOG_INPUTS 6
#endif

// define additional symbols when there are 8 analog iputs
#if (NUM_ANALOG_INPUTS == 8)
#define PIN_A6 (20)
#define PIN A7 (21)
static const uint8_t A6 = PIN_A6;
static const uint8_t A7 = PIN_A7;
#endif
```

That way they only exist when they should and sketch could actually use something like
`# ifdef PIN_A6`

to determine if there is an analog pin A6 or not.
Same for A7

Start with the standard and eightanaloginputs pins_arduino.h files and compare how NUM_ANALOG_INPUTS is defined and consumed. Confirm that the standard variant exposes only A0–A5 while the eightanaloginputs variant exposes A6 and A7 as well. Done means sketches can use PIN_A6 or PIN_A7 conditionally without those symbols existing on six-input hardware.

Written by the indexing model from the issue text.