Fix webhook username mismatch

Fixing variable name mismatch so that the Authorization HTTP header is added to webhook notifications.

Use HTTPS instead of git to download tether-drop package

git protocol (tcp/udp/9418) is forbidden when using some corporate
proxies. HTTPS is used more widely.

feat(plugins): made notifiers more pluggable and easier to support many of them, new ones can now be added without modifying any existing file, #7162

Merge branch 'notifier-telegram-messenger' of https://github.com/leonoff/grafana into leonoff-notifier-telegram-messenger

* tech(build): replace npm with yarn

* tech(build): change node version for CI

* Org update should give error if not found

* Used affected rows

fix(templating): fixed issue with experimental feature template variable value groups tags, fixes #6752