insecure-skip-tlog-verify: rename and adapt the cert expiration check

[hectorj2f]

closes: #2590

Summary

Based on the discussion in #2590, we decided to rename the flag from insecure-skip-tlog-verify to insecure-ignore-tlog. Likewise, we also decided to throw an error expecting a signed timestamp whenever users ignore the tlog validation and no timestamp-rfc3161 was present.

Release Note

Documentation

[codecov-commenter]

Codecov Report

Merging #2620 (f3fca6f) into main (3a492c7) will decrease coverage by 0.00%.
The diff coverage is 25.00%.

@@            Coverage Diff             @@
##             main    #2620      +/-   ##
==========================================
- Coverage   30.14%   30.13%   -0.01%     
==========================================
  Files         146      146              
  Lines        9252     9255       +3     
==========================================
  Hits         2789     2789              
- Misses       6032     6035       +3     
  Partials      431      431              

Impacted Files	Coverage Δ
cmd/cosign/cli/options/verify.go	0.00% <0.00%> (ø)
cmd/cosign/cli/verify.go	0.00% <0.00%> (ø)
cmd/cosign/cli/verify/verify.go	22.49% <0.00%> (ø)
cmd/cosign/cli/verify/verify_attestation.go	3.43% <0.00%> (ø)
pkg/cosign/verify.go	39.36% <20.00%> (-0.14%)	⬇️
cmd/cosign/cli/verify/verify_blob.go	49.33% <100.00%> (ø)
cmd/cosign/cli/verify/verify_blob_attestation.go	33.64% <100.00%> (ø)

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[priyawadhwa]

lgtm!

[haydentherapper]

just a comment nit, looks good!