[go-completer] update gopls version and support go install

when build YCM with --go-completer, it's building error with following:

go get: installing executables with 'go get' in module mode is deprecated.
	To adjust and download dependencies of the current module, use 'go get -d'.
	To install using requirements of the current module, use 'go install'.
	To install ignoring the current module, 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'.

My go version

go version
go version go1.17.1 linux/amd64

the document said:
https://github.com/golang/tools/blob/master/gopls/doc/advanced.md#installing-unreleased-versions

# For go1.17 or older, the above `go get` command will build and
# install `gopls`. For go1.18+ or tip, run the following command to install
# using selected versions in go.mod.
go install golang.org/x/tools/gopls

However, it would be better to support new command as go install

To solve this problem for me change get -> install but it would break backward-compatible:

ycmd/build.py

Line 830 in a7b972b

CheckCall( [ go, 'get', 'golang.org/x/tools/gopls@v0.7.1' ],

it would break backward-compatible

Exactly why we haven't done that yet. Latest ubuntu LTS is only at 1.14 and that one won't work with go install.