Implement Calibrate stub for ADC2 and ADC3

[harryjwright]

Currently, Adc::new has a trait bound for Calibrate. While ADC1 has a full calibration implementation for internal signals, ADC2 and ADC3 do not.

This PR adds the missing stub implementations:

impl Calibrate for Adc<pac::ADC2> {} (gated by adc2 feature)

impl Calibrate for Adc<pac::ADC3> {} (gated by adc3 feature)

This allows users to use the standard Adc::new constructor for these peripherals instead of manual struct initialization.

[burrbull]

Add change to Changelog please

[harryjwright]

Changelog updated! Thanks for the catch.

[linuxtim]

git log entry for the master branch includes:

commit 172ccc0fa26a071b04a5f4de61cfb9d73adc7c98
Author: harryjwright <[email protected]>
Date:   Sun Jan 11 03:23:25 2026 +0000
    
    Add changelog entry for [#YOUR_PR_NUMBER]

Not sure if/how you want to fix this up? More generally it might be better to include the changelog commit into the code commit (i.e. rebase)?