Add a section for missing pet upgrades

[Jacob385]

The site already shows missing pets, but if a player has a pet that is not at the highest rarity it can be hard to tell they are missing that upgrade.

So let's add a section for missing pet upgrades. It can look like the section for missing accessory upgrades. When a player has a pet that has a higher rarity available (not max rarity) it will be listed in this section.

This addition will allow players to manage their pet collection better and see ways to progress it. This is especially useful because not all pets have the same max rarity usually maxing out at either LEGENDARY or MYTHIC.

[github-actions]

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