Add deprecation warning for sget CLI and packages

[imjasonh]

Signed-off-by: Jason Hall [email protected]

@dlorenc @lukehinds

Release Note

The sget tool in the cosign repo is deprecated and will be removed in a future release -- see https://github.com/sigstore/sget for the new tool.

[codecov-commenter]

Codecov Report

Merging #2019 (cbdb114) into main (4e282fd) will decrease coverage by 0.02%. The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2019      +/-   ##
==========================================
- Coverage   26.15%   26.13%   -0.03%     
==========================================
  Files         127      127              
  Lines        7425     7432       +7     
==========================================
  Hits         1942     1942              
- Misses       5228     5235       +7     
  Partials      255      255              

Impacted Files	Coverage Δ
cmd/cosign/cli/policy_init.go	1.38% <ø> (-0.01%)	:arrow_down:
pkg/cosign/keys.go	61.36% <0.00%> (ø)
pkg/cosign/common.go	0.00% <0.00%> (ø)
cmd/cosign/cli/sign.go	0.00% <0.00%> (ø)
cmd/cosign/cli/clean.go	0.00% <0.00%> (ø)
cmd/cosign/cli/attest.go	0.00% <0.00%> (ø)
cmd/cosign/cli/commands.go	0.00% <0.00%> (ø)
cmd/cosign/cli/signblob.go	0.00% <0.00%> (ø)
pkg/cosign/rekor_factory.go	0.00% <0.00%> (ø)
... and 6 more

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[imjasonh]

Is this good to merge? 🙏