Change GA4 report dimension from `pagePath` to `pagePathPlusQueryString` if/when compatible.

## Feature Description

Following on from the change [here](https://github.com/google/site-kit-wp/pull/6422) to switch from `pagePathPlusQueryString` to `pagePath` (see the PR description for details), if/when `pagePathPlusQueryString` is made more compatible and supports the various metrics/dimensions that we need to combine it with, we should revert to `pagePathPlusQueryString` in order to better support sites which don't use pretty permalinks.

Note that we might want to switch to `pagePathPlusQueryString` only for sites which are not using pretty permalinks, and keep using `pagePath` for permalink-enabled sites, as `pagePath` would prevent unrelated query params from impacting the per-page report filtering.

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## Acceptance criteria

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## Implementation Brief

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### Test Coverage

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## QA Brief

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## Changelog entry

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Start by reading PR 6422 and its description to understand the current switch from pagePathPlusQueryString to pagePath. Investigate whether GA4 now supports pagePathPlusQueryString with the required metrics and dimensions, including the pretty-permalink distinction. Done means selecting a compatible dimension and validating the affected per-page reports.

Written by the indexing model from the issue text.