Upgrade Go to 1.21.1

CONTRIBUTING.md

Dockerfile

appveyor.yml

commands/scp_unix.go

drivers/virtualbox/vtx_intel.go

drivers/virtualbox/vtx_other.go

drivers/vmwarefusion/fusion.go

libmachine/drivers/plugin/localbinary/plugin.go

libmachine/provision/provisiontest/sshcommander.go

libmachine/shell/shell.go

libmachine/shell/shell_unix_test.go

mk/main.mk

mk/validate.mk