chore: upgrade go to v1.21.6 due to vuls

[ericzzzzzzz]

Fixes: #nnn
Related: Relevant tracking issues, for context
Merge before/after: Dependent or prerequisite PRs

Description

User facing changes (remove if N/A)

Follow-up Work (remove if N/A)

[codecov]

Codecov Report

Attention: 295 lines in your changes are missing coverage. Please review.

Comparison is base (290280e) 70.48% compared to head (d677069) 63.60%.
Report is 1112 commits behind head on main.

Files	Patch %	Lines
cmd/skaffold/app/cmd/exec.go	16.32%	40 Missing and 1 partial ⚠️
cmd/skaffold/app/cmd/filter.go	47.27%	22 Missing and 7 partials ⚠️
cmd/skaffold/app/cmd/lsp.go	28.12%	23 Missing ⚠️
cmd/skaffold/app/cmd/verify.go	23.33%	23 Missing ⚠️
cmd/skaffold/app/cmd/fix.go	51.16%	17 Missing and 4 partials ⚠️
cmd/skaffold/app/cmd/inspect_job_manifest_paths.go	60.00%	15 Missing and 1 partial ⚠️
cmd/skaffold/app/cmd/inspect_namespaces.go	50.00%	13 Missing and 1 partial ⚠️
...md/skaffold/app/cmd/inspect_config_dependencies.go	45.83%	12 Missing and 1 partial ⚠️
cmd/skaffold/app/cmd/lint.go	42.85%	12 Missing ⚠️
cmd/skaffold/app/cmd/inspect_build_env.go	60.71%	11 Missing ⚠️
... and 21 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9303      +/-   ##
==========================================
- Coverage   70.48%   63.60%   -6.89%     
==========================================
  Files         515      635     +120     
  Lines       23150    32773    +9623     
==========================================
+ Hits        16317    20844    +4527     
- Misses       5776    10325    +4549     
- Partials     1057     1604     +547     

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