chore: address `io/ioutil` depreciation

[tenthirtyam]

io/ioutil is deprecated as of Go 1.16:

• Updates ioutil.Discard to io.Discard.
• Updates ioutil.TempFile to os.CreateTemp.
• Updates ioutil.ReadFile to os.ReadFile.
• Updates ioutil.WriteFile to os.WriteFile.

packer-plugin-vsphere on  chore/ioutil-depreciation [!] via 🐹 v1.21.6 took 29.6s 
✦ ➜ make build

packer-plugin-vsphere on  chore/ioutil-depreciation [!] via 🐹 v1.21.6 took 8.7s 
✦ ➜ make test
?       github.com/hashicorp/packer-plugin-vsphere      [no test files]
?       github.com/hashicorp/packer-plugin-vsphere/builder/vsphere/common/testing       [no test files]
?       github.com/hashicorp/packer-plugin-vsphere/builder/vsphere/examples/driver      [no test files]
?       github.com/hashicorp/packer-plugin-vsphere/version      [no test files]
ok      github.com/hashicorp/packer-plugin-vsphere/builder/vsphere/clone        3.255s
ok      github.com/hashicorp/packer-plugin-vsphere/builder/vsphere/common       9.730s
ok      github.com/hashicorp/packer-plugin-vsphere/builder/vsphere/driver       8.574s
ok      github.com/hashicorp/packer-plugin-vsphere/builder/vsphere/iso  7.139s
ok      github.com/hashicorp/packer-plugin-vsphere/builder/vsphere/supervisor   15.838s
ok      github.com/hashicorp/packer-plugin-vsphere/post-processor/vsphere       9.135s
ok      github.com/hashicorp/packer-plugin-vsphere/post-processor/vsphere-template      11.109s