Update Go tests to target single version

In some projects, it's best practice to test in multiple versions, per our Go development guidelines, for these reasons:

• When a new version of Go is released, we should start integrating it into the CI pipelines to verify forward compatibility.
• To enable backports, we must support the versions of Go shipped by Distribution in the latest 3 minor GitLab releases, excluding the active milestone.

We currently test our Go code in Go v1.23 and v1.24, but now that we've been running Go code in production for a few releases, it seems like we can simplify things here and drop test coverage for versions we're not actively using. The reasons above don't really apply to this project, where our Go code is relatively simple and not distributed in like GitLab (we distribute our compiled Dockerized site, which does not need a Go runtime to use).

Related: !545 (comment 2433315488)

Done when

• Local dev environments and CI jobs only target one Go version (v1.24.x)
• Dependency update docs updated accordingly