feat: Change go get to go install in README

[kavfixnel]

go get is no longer a valid command outside of the module:

~ ❯ go get -u moul.io/assh/v2
go: go.mod file not found in current directory or any parent directory.
	'go get' is no longer supported outside a module.
	To build and install a command, use 'go install' with a version,
	like 'go install example.com/cmd@latest'
	For more information, see https://golang.org/doc/go-get-install-deprecation
	or run 'go help get' or 'go help install'.

Instead, we should be using go install (See https://go.dev/doc/go-get-install-deprecation). Reflect this in the README

[kavfixnel]

I just noticed that there is also a tested with Go1.7 or above note right below this, that will likely be incorrect with this new command. Is go 1.7+ support very important, or can we assume that most people will have go 1.17+?

[moul]

We can definitely move to go1.17+ 👍.

[github-actions]

🎉 This PR is included in version 2.16.0 🎉

The release is available on GitHub release

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