upgrade .go-version to 1.26.6

this ensures all *production* binaries (not e2e test images) shipped with the release are compiled with 1.26.6, which contains CVE fixes

https://groups.google.com/d/msgid/golang-announce/4f84cf61.EAAACRzWXsgAAAAAAAAAA-p9MGAAAYKKSQYAAAAAADE8OwBqfhLk%40mailjet.com