Update readme markdown field in catalog resource version

This MR is just a refactoring so there should be no UI changes.

Summary of changes:

• updates readme field in Ci::Catalog::Resources::Version to use markdown_field helper
• adds cached_markdown_version , readme and readme_html to improve performance
• removes unused code & redundant comments

ci: == [advisory_lock_connection] object_id: 124960, pg_backend_pid: 59875
ci: == 20240503155548 AddCachedMarkdownFieldsToVersion: migrating =================                                                                                                                                        ci: -- add_column(:catalog_resource_versions, :cached_markdown_version, :integer, {:null=>true})
ci:    -> 0.0016s
ci: -- add_column(:catalog_resource_versions, :readme, :text, {:null=>true})
ci:    -> 0.0004s
ci: -- add_column(:catalog_resource_versions, :readme_html, :text, {:null=>true})
ci:    -> 0.0005s
ci: == 20240503155548 AddCachedMarkdownFieldsToVersion: migrated (0.0452s) ========

ci: == [advisory_lock_connection] object_id: 124960, pg_backend_pid: 59875