refactor: apply Cosmos SDK v0.50 conventions

[hallazzang]

Description

The main changes are:

• Use Walk() instead of old iterator-based approaches where applicable
• Return an error only instead of a bit of redundant (T, bool error)
• Remove unused storeService references in keepers

This PR introduces no state-breaking changes but note that merging this PR is not urgent.

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[RiccardoM]

Can you please add back these tests by changing them to make sure the returned error is ErrNotFound? We might need to create a new testutils method to do this

[hallazzang]

err == nil means the pool was found, so we don't need further tests after suite.Require().NoError(err). I added suite.Require().ErrorIs(err, collections.ErrNotFound) to where we originally had suite.Require().False(found) tests.