Fix Git.version metric value generated causes bug in Version app

What does this MR do and why?

This MR adds to_json method to VersionInfo. This fixes the bug when git.version metric (from ServicePing) being incorrectly represented using method to_s when payload is generated.

[16] pry(main)> payload = Gitlab::Usage::ServicePingReport.for(output: :all_metrics_values)
[17] pry(main)> payload[:git].to_json
=> "{\"version\":{\"major\":2,\"minor\":35,\"patch\":1}}"
[18] pry(main)> Gitlab::Json.pretty_generate(payload[:git])
=> "{\n  \"version\": \"2.35.1\"\n}"

How to set up and validate locally

pry(main)> payload = Gitlab::Usage::ServicePingReport.for(output: :all_metrics_values)
pry(main)> Gitlab::Json.pretty_generate(payload[:git])
=> "{\"version\":{\"major\":2,\"minor\":35,\"patch\":1}}"

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #359925 (closed)