Get rid of old /top-stats endpoint

[RobertJoonas]

Changes

Could have been done sooner (with the actual endpoint switch) but I mistakenly thought it's needed for Dashboard CSV export. Which it's not.

The top_stats_test.exs probably gives the query API v2 a good amount of extra coverage. This PR modernizes that file to keep that coverage:

• Query the new endpoint instead, where applicable.
• Tests such as those asserting on the metric selection based on query state, or a returned graph_metric key were removed
• Instead of asserting on with_imported_switch_info, we now assert on query.skip_imports_reason and query.imports_included in all the same scenarios.
• Some more basic tests, such as "counts unique visitors" or "counts pageviews" were also removed.

Rewriting the tests uncovered two issues:

1. [unrelated to top stats migration] - when scroll depth is queried alone, and with imported data included, the native query aggregation doesn't work (out of scope here, added a ticket on basecamp)
2. time on page is returned as nil (instead of 0 as before) when it cannot be calculated. For users it means that Top Stats will simply hide the metric. I've updated the test to assert on nil.

Tests

• Automated tests have been added

Changelog

• This PR does not make a user-facing change

Documentation

• This change does not need a documentation update

Dark mode

• This PR does not change the UI