win32: Drop support for MSVC 2003

[k-takata]

The patch 8.2.4199 actually dropped support for MSVC 6.0.
This PR drops support for MSVC 2002/2003.

[codecov]

Codecov Report

Merging #9623 (9804fcd) into master (7e765a3) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9623      +/-   ##
==========================================
- Coverage   83.51%   83.51%   -0.01%     
==========================================
  Files         154      154              
  Lines      174516   174602      +86     
  Branches    39260    39271      +11     
==========================================
+ Hits       145748   145815      +67     
- Misses      16670    16680      +10     
- Partials    12098    12107       +9     

Flag	Coverage Δ
huge-clang-none	81.89% <ø> (+0.01%)	⬆️
huge-gcc-none	82.25% <ø> (+0.01%)	⬆️
huge-gcc-testgui	80.70% <ø> (-0.01%)	⬇️
huge-gcc-unittests	2.03% <ø> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/sound.c	85.57% <0.00%> (-2.89%)	⬇️
src/if_xcmdsrv.c	76.88% <0.00%> (-0.53%)	⬇️
src/netbeans.c	72.05% <0.00%> (-0.37%)	⬇️
src/normal.c	90.50% <0.00%> (-0.23%)	⬇️
src/gui_gtk_x11.c	57.07% <0.00%> (-0.10%)	⬇️
src/syntax.c	79.54% <0.00%> (-0.07%)	⬇️
src/channel.c	83.66% <0.00%> (-0.05%)	⬇️
src/version.c	83.82% <0.00%> (ø)
src/move.c	89.28% <0.00%> (+<0.01%)	⬆️
src/eval.c	90.81% <0.00%> (+0.01%)	⬆️
... and 8 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7e765a3...9804fcd. Read the comment docs.