build: Fix changelog script to display the tag creation date

[steve-marmalade]

Signed-off-by: Steve McClain steve@marmalade.co

Checklist:

• My organization is added to USERS.md.

This is a very small quality-of-life fix to the hack/changelog.sh script, discussed with @alexec in slack. It now shows the date the tag was created instead of the date of the tagged commit.

[codecov]

Codecov Report

Merging #6510 (fdbe2a0) into master (8d60296) will increase coverage by 0.09%.
The diff coverage is 87.87%.

❗ Current head fdbe2a0 differs from pull request most recent head ba96aed. Consider uploading reports for the commit ba96aed to get more accurate results

@@            Coverage Diff             @@
##           master    #6510      +/-   ##
==========================================
+ Coverage   48.49%   48.58%   +0.09%     
==========================================
  Files         260      260              
  Lines       18804    18827      +23     
==========================================
+ Hits         9119     9148      +29     
+ Misses       8678     8671       -7     
- Partials     1007     1008       +1     

Impacted Files	Coverage Δ
util/kubeconfig/kubeconfig.go	31.01% <87.87%> (+11.50%)	⬆️
cmd/argo/commands/get.go	59.18% <0.00%> (+0.29%)	⬆️
cmd/argoexec/commands/emissary.go	51.79% <0.00%> (+1.43%)	⬆️

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