Update to github.com/shirou/gopsutil v2.20.5

[danielnelson]

closes #7018
closes #6807

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

[ssoroka]

👍 but test failures. plugins/inputs/procstat/native_finder_windows.go:63:104: undefined: process.Win32_Process

[danielnelson]

Here is the before and after for the native finder. CPU is down on Windows but allocations are up.

$ go test -short ./plugins/inputs/procstat/... -bench=. -benchmem
goos: windows
goarch: amd64
pkg: github.com/influxdata/telegraf/plugins/inputs/procstat
BenchmarkPattern-16                   18          68615033 ns/op           27202 B/op        290 allocs/op
BenchmarkFullPattern-16               31          34742177 ns/op           10256 B/op        101 allocs/op
PASS
ok      github.com/influxdata/telegraf/plugins/inputs/procstat  3.443s

dbn@LOKI C:\Users\dbn\go\src\github.com\influxdata\telegraf
$ go test -short ./plugins/inputs/procstat/... -bench=. -benchmem
goos: windows
goarch: amd64
pkg: github.com/influxdata/telegraf/plugins/inputs/procstat
BenchmarkPattern-16                   18          62448017 ns/op           24564 B/op        281 allocs/op
BenchmarkFullPattern-16              444           2551948 ns/op           92771 B/op       1105 allocs/op
PASS
ok      github.com/influxdata/telegraf/plugins/inputs/procstat  3.514s

This may fix #6807 and the removes the need for #7129