Bump gopsutil to v2.20.6 (and resolve LXC container uptime monitoring)

[citruspi]

Required for all PRs:

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

This PR bumps the gopsutil dependency to version v2.20.6 to include the fix for LXC guest detection which in turn resolves the open bug for LXC container uptime monitoring.

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master

$ ./telegraf --test --input-filter system --config foo.conf | grep 'uptime='
2020-07-09T23:34:47Z I! Starting Telegraf 
> system,host=telegraf-lxc-test uptime=891624i 1594337688000000000

branch

$ ./telegraf --test --input-filter system --config foo.conf | grep 'uptime='
2020-07-09T23:34:50Z I! Starting Telegraf 
> system,host=telegraf-lxc-test uptime=2291i 1594337691000000000