Upgrade to golang 1.21

[jkl73]

Clean up go.mod and upgrade dependencies.
Update all Go version to 1.21.

Changes made to accommodate linter for deprecation errors:

1. Update cmd/flush.go `cmd/pubkey.go

cobra.ExactValidArgs(1) -> cobra.MatchAll(cobra.ExactArgs(1), cobra.OnlyValidArgs)

2. Add a comment in server/import.go to suppress a linter error (because elliptic.GenerateKey is deprecated, but the new library crypto/ecdh doesn't support P226)
3. Update launcher/auth.go (options.Authrorize is deprecated)

options.Authorizer = docker.NewDockerAuthorizer(authOpts...) -> options.Hosts = docker.ConfigureDefaultRegistries(
		docker.WithAuthorizer(docker.NewDockerAuthorizer(authOpts...)))

[jkl73]

/gcbrun