importing module that requires go 1.21 (slices package)

[KEINOS]

• Error log during CI:

  • https://github.com/KEINOS/go-gisty/actions/runs/6599375150/job/17928328976#step:3:77

    slices: package slices is not in GOROOT (/opt/hostedtoolcache/go/1.20.10/x64/src/slices)
    note: imported by a module that requires go 1.21
    make: *** [Makefile:22: update] Error 1
    Error: Process completed with exit code 2.
    

  • Reason:
    • The golang.org/x/exp/slices package goes official as slices from Go 1.21
    • New slices package | Go 1.21 Release Notes @ tip.golang.org

• Possible fix:

  • Upgrade to Go 1.21
  • Reason:
    • The cli/cli module we depend on has updated the minimum version of Go to 1.21. So we should do the same.
    • Upgrade to Go 1.21 cli/cli#7843