Backport of Reduce number of places where go version is set into release/1.12.x

[hc-github-team-secure-vault-core]

Backport

This PR is auto-generated from #17762 to be assessed for backporting due to the inclusion of the label backport/1.12.x.

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The below text is copied from the body of the original PR.

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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

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Overview of commits

• 81a563d

[hashicorp-cla]

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