chore: update go version to 1.21

[werne2j]

Description

Update go version to 1.21

Fixes:
https://github.com/argoproj-labs/argocd-vault-plugin/security/dependabot/37 & https://github.com/argoproj-labs/argocd-vault-plugin/security/dependabot/36

Checklist

Please make sure that your PR fulfills the following requirements:

• Reviewed the guidelines for contributing to this repository
• The commit message follows the Conventional Commits Guidelines.
• Tests for the changes have been updated
• Are you adding dependencies? If so, please run go mod tidy -compat=1.17 to ensure only the minimum is pulled in.
• Docs have been added / updated
• Optional. My organization is added to USERS.md.

Type of Change

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• New tests
• Build/CI related changes
• Documentation content changes
• Other (please describe)

Other information

[codecov-commenter]

Codecov Report

Merging #565 (e67a3b7) into main (5084eca) will not change coverage.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #565   +/-   ##
=======================================
  Coverage   71.29%   71.29%           
=======================================
  Files          26       26           
  Lines        1951     1951           
=======================================
  Hits         1391     1391           
  Misses        460      460           
  Partials      100      100           

Files	Coverage Δ
cmd/util.go	76.92% <100.00%> (ø)
pkg/auth/vault/kubernetes.go	64.70% <100.00%> (ø)
pkg/utils/util.go	59.21% <100.00%> (ø)

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[jkayani]

What is this for?

[werne2j]

Issue with the Linux build https://github.com/argoproj-labs/argocd-vault-plugin/actions/runs/6526155961/job/17719605595

[jkayani]

LGTM just 1 question