Fix import path for v7 (#800)

* fix import path for v7

find ./ -name "*.go" | xargs sed -i -e 's|"github.com/oauth2-proxy/oauth2-proxy|"github.com/oauth2-proxy/oauth2-proxy/v7|'

* fix module path

* go mod tidy

* fix installation docs

* update CHANGELOG

* Update CHANGELOG.md

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>