Migrate from Query::single and friends to Single

[pablo-lua]

Objective

• closes Replace all uses of Query::single with Single #15866

Solution

• Simply migrate where possible.

Testing

• Expect that CI will do most of the work. Examples is another way of testing this, as most of the work is in that area.

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Notes

For now, this PR doesn't migrate QueryState::single and friends as for now, this look like another issue. So for example, QueryBuilders that used single or World::query that used single wasn't migrated. If there is a easy way to migrate those, please let me know.

Most of the uses of Query::single were removed, the only other uses that I found was related to tests of said methods, so will probably be removed when we remove Query::single.

[alice-i-cecile]

I've had to open this up in Codespaces to review it, as Github's webpage hates PRs with many files changed 🙃 That said, looks great! I particularly appreciate the use of Option<Single> in tests to ensure that we're still panicking if things aren't found.

[rparrett]

Guessing this is going to have conflicts after #15857. Can take a look after those get fixed up.

[pablo-lua]

@BenjaminBrienen All comments addressed.