Reduce number of places where go version is set

[ncabatoff]

Now we have only .go_version and @executors.yml. Also add a GH action to ensure those two are kept in sync.

Details:

• go_test accepted a go_image parameter but nothing ever specified it, we were just relying on the default, so now instead we'll use the GO_IMAGE environment variable
• reworked @executors to look more like the ent version of the file; ensure that GO_IMAGE environment variable is set everywhere; and set the version once in an anchor, in references.environment.GO_IMAGE
• add a .go-version file in the root of the tree, as some ecosystem repos are doing already, and change the Makefile to use that file instead of hard-coding it; this allows us to programatically determine the current version the repo wants without having to parse the Makefile or invoke make
• remove references to specific Go version from README; we can assume that most people will install the latest Go version, and the Makefile will protect against those who don't
• remove references to specific Go version in dockerfiles; the Makefile target that uses them already passes in the Go version anyway

[tomhjp]

Some optional suggestions on the action, but this seems like a nice improvement 👍