Don't lowercase input image refs, just fail

[imjasonh]

Reverts #1409
Fixes #1568

Summary

Lowercasing the input was supposed to help when the user's requested repo name included uppercase letters (e.g., ghcr.io/MyUser/CAPS-REPO), which isn't allowed. This unintentionally also lowercased the tag component, which is allowed to contain uppercase letters, and lowercasing them breaks the user's expectation.

Release Note

Don't lowercase the input image ref, just fail if it includes uppercase characters

[codecov-commenter]

Codecov Report

Merging #1586 (adb92ce) into main (c341168) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #1586      +/-   ##
==========================================
- Coverage   27.19%   27.15%   -0.04%     
==========================================
  Files         136      136              
  Lines        7712     7712              
==========================================
- Hits         2097     2094       -3     
- Misses       5386     5388       +2     
- Partials      229      230       +1     

Impacted Files	Coverage Δ
cmd/cosign/cli/sign/sign.go	1.62% <0.00%> (ø)
pkg/cosign/tuf/client.go	62.34% <0.00%> (-0.95%)	⬇️

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