Add cached markdown fields for ML Model Version

What does this MR do and why?

This MR adds the cached markdown fields to the Ml::ModelVersion so it can later be used with markdown descriptions.

Migration up

bin/rails db:migrate:up:main VERSION=20240812092309
main: == [advisory_lock_connection] object_id: 127060, pg_backend_pid: 53428
main: == 20240812092309 AddCachedMarkdownToMlModelVersion: migrating ================
main: -- change_table(:ml_model_versions)
main:    -> 0.0016s
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- execute("ALTER TABLE ml_model_versions\nADD CONSTRAINT check_4d50116294\nCHECK ( char_length(description_html) <= 50000 )\nNOT VALID;\n")
main:    -> 0.0008s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0001s
main: -- execute("ALTER TABLE ml_model_versions VALIDATE CONSTRAINT check_4d50116294;")
main:    -> 0.0003s
main: -- execute("RESET statement_timeout")
main:    -> 0.0001s
main: == 20240812092309 AddCachedMarkdownToMlModelVersion: migrated (0.0267s) =======

main: == [advisory_lock_connection] object_id: 127060, pg_backend_pid: 53428

Migration down

bin/rails db:migrate:down:main VERSION=20240812092309
main: == [advisory_lock_connection] object_id: 127060, pg_backend_pid: 52738
main: == 20240812092309 AddCachedMarkdownToMlModelVersion: reverting ================
main: -- remove_columns(:ml_model_versions, :cached_markdown_version, :description_html)
main:    -> 0.0011s
main: == 20240812092309 AddCachedMarkdownToMlModelVersion: reverted (0.0038s) =======

main: == [advisory_lock_connection] object_id: 127060, pg_backend_pid: 52738```

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Related to #478475 (closed)