Remove Elastic::V12P1 files

What does this MR do and why?

Removes Elastic::V12P1 files. The main change is changing https://gitlab.com/gitlab-org/gitlab/-/blob/180a956ddc13ede9716a2ae60d575b1017859637/ee/lib/elastic/multi_version_util.rb#L11 from "V12p1" to "Latest".

• ✅ es_import calls r.__elasticsearch__.version(version_namespace) which points to "Latest" and still works (only used for indexing snippets)
• ✅ Gitlab::Elastic::Helper.default calls Repository.__elasticsearch__.version(:Elastic::MultiVersionUtil::TARGET_VERSION) which now uses Latest instead of V12p1
• ✅ GemExtensions::Elasticsearch::Model::Adapter::ActiveRecord::Importing points to model.__elasticsearch__.version(version_namespace). I'm not sure how and if that's used but version_namespace is "Latest" so it should be good
• ✅ All tests pass

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. Run Snippet.es_import
2. See if your client is correctly being built: Gitlab::Elastic::Helper.default.version is "Latest". On master it is V12p1.