Standardise the usage of External Icons in all links and buttons

## Feature Description

Currently, the use of the external link icon across Site Kit is inconsistent, resulting in a fragmented user experience that lacks clear visual logic. This issue tracks the work to establish a unified standard for when external icons should and should not appear on links and buttons throughout the plugin.

The new standard shifts the icon's role from a purely technical indicator (i.e. "opens in a new tab") to a contextual one. External icons should be reserved exclusively for Cross-Product links — those directing users to take action on another Google/third-party service platform (e.g. Search Console, Analytics, RRM) — and removed from educational, support, data, and WP environment links where they add visual noise without meaningful context.

For background discussion, see: [Slack thread 1](https://fueled.slack.com/archives/C01CA828ZPZ/p1772382101556049) and [Slack thread 2](https://fueled.slack.com/archives/C01CA828ZPZ/p1773140271358249).

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## Acceptance criteria

* External icons **should be** shown only on Cross-Product links — those directing the user to take an action on another Google or third-party service platform. This includes:
* "Source: [Service]" attribution links below each dashboard widget on the Main and Entity Dashboards (destination: the service platform, e.g. Analytics, Search Console)
* "See full details in [Service]" links in service-specific settings within Module Settings (destination: the service platform)
* "Edit in [Service]" links in service-specific Module settings (destination: the service platform)
* RRM configuration CTAs (e.g. "Complete publication setup") in notices (destination: RRM platform)
* "Learn more about… in [Service]" links in informative widget text (destination: service documentation)
* "Google privacy policy" links in informative widget text (destination: Google privacy policy page)
* External icons **should not** be shown on:
* Educational "Learn more" links in banners, setup CTAs, notices, settings, callouts, and all other appearances (destination: Site Kit documentation)
* Support "Get help" links in warning/error notices (destination: Site Kit documentation)
* Data links — site page/post name or URL — in content widgets (Top Search Queries, Top Content, Earning Content) and entity dashboard headers (destination: Search Console, Analytics, or the site page itself)
* "Visit WordPress settings" links in notices (destination: WP settings)
* External icons **should not** be shown on links within a list (e.g. pages links list) on Dashboard and Entity Dashboard widgets, regardless of destination.

## Implementation Brief

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### Test Coverage

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## QA Brief

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## Changelog entry

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