fix(cli): move deprecation warnings to stderr

[Tieske]

This is ugly and inconsistent, but not breaking.

[codecov-commenter]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (d110df8) 32.87% compared to head (6699a4d) 32.87%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1075   +/-   ##
=======================================
  Coverage   32.87%   32.87%           
=======================================
  Files         103      103           
  Lines       12847    12847           
=======================================
  Hits         4224     4224           
  Misses       8214     8214           
  Partials      409      409           

Files	Coverage Δ
cmd/gateway_diff.go	0.00% <0.00%> (ø)
cmd/gateway_dump.go	0.00% <0.00%> (ø)
cmd/gateway_ping.go	0.00% <0.00%> (ø)
cmd/gateway_reset.go	0.00% <0.00%> (ø)
cmd/gateway_sync.go	0.00% <0.00%> (ø)
cmd/gateway_validate.go	0.00% <0.00%> (ø)
cmd/file_convert.go	0.00% <0.00%> (ø)

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