Sync with upstream AVR, minor fixes

[vintagepc]

Description

• Fixes incorrect pulley steps/mm in MMU
• Removes obsolete serial hackery now that the bug has been fixed upstream.

[codecov]

Codecov Report

Merging #365 (dab84dd) into master (41b23ee) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #365      +/-   ##
==========================================
- Coverage   91.49%   91.47%   -0.02%     
==========================================
  Files         167      167              
  Lines        7846     7828      -18     
==========================================
- Hits         7179     7161      -18     
  Misses        667      667              

Impacted Files	Coverage Δ
parts/printers/Prusa_MK2_13.cpp	94.40% <ø> (-0.34%)	⬇️
parts/printers/Prusa_MK2_13.h	100.00% <ø> (ø)
parts/printers/Prusa_MK3S.cpp	98.24% <ø> (-0.13%)	⬇️
parts/printers/Prusa_MK3S.h	100.00% <ø> (ø)
parts/boards/MM_Control_01.cpp	100.00% <100.00%> (ø)

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