Use page version's `updated_at` timestamp as cache key

[mamhoff]

What is this pull request for?

Currently, it's hard for us to invalidate caches for Alchemy pages when content that's referenced through ingredients with related objects changes.

For example, in a situation where a user combines Alchemy and Solidus using the alchemy_solidus gem, a page's cache key does not update when a product that's referenced through a SpreeProduct ingredient changes.

There's a PR up on the alchemy_solidus gem that touches ingredients in these situations, and with this change, that touching can be used for breaking caches. [1]

1

Checklist

• [x] I have followed Pull Request guidelines
• [x] I have added a detailed description into each commit message
• [x] I have added tests to cover this change

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.93%. Comparing base (f6b6ad5) to head (6787e96). Report is 20 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2862 +/- ## ========================================== + Coverage 95.86% 95.93% +0.07% ========================================== Files 229 229 Lines 6211 6224 +13 ========================================== + Hits 5954 5971 +17 + Misses 257 253 -4 ```

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