Fixes for prefetch related and saving related

0.20.1

✨ Breaking changes

• Note that this is the first non-beta release of ormar with support for Pydantic v2. Check release notes for 0.20.0 and https://collerek.github.io/ormar/0.20.0b1/migration/

🐛 Fixes

• Fix merging same target models when using select_related with prefetch_related #906
• Fix saving related with pk only models #812
• Fix adding the same relation multiple times corrupting relation cache #1335

✨ Features

• Allow adding indexed on foreign keys by @cmflynn, thanks! #1276

💬 Other

• Some docs fixes by @Chaoyingz, thanks!