ci: Add Go 1.23 to test matrix

Go 1.23 was released (https://tip.golang.org/doc/go1.23). This version adds many nice perks, which are useful to Gitaly.

The testing version was added to the build images: gitlab-build-images!844 (merged)

It's worth noted that the minimum Git version of the image was bumped to 2.45. This version aligns with our minimum supported Git version. gitlab-build-images!835 (merged)

Other changes in the .gitlab-ci.yml are from the YAML linter. We never run them before.