Make /government/get-involved an exact route

[KludgeKML]

• The prefix route seems to be a hangover from when whitehall was rendering the Take Part items. These all now have their own content item, so anything under get-involved is either the get involved item or has its own route.
• Both of the SpecialRoute instances are now exact path, so make that the default in rake task and test, and remove it from the instances.

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[kevindew]

Code looks good to me, so assuming you've validated that there aren't risks in this changing this looks good to me.

Good luck with fixing the test though that looked a little confusing

[KludgeKML]

The only routes under /government/get-involved are the take-part pages (all linked to from the main get-involved page), which all have their own content items and routes (https://github.com/alphagov/frontend/blob/main/config/routes.rb#L82-L85), so I'm fairly confident that this won't have any knock-on effects.

[KludgeKML]

I think that test might be a known flaky one (it passed on re-run), but I might double-check with the Whitehall team to confim.

[KludgeKML]

Have confirmed all relevant paths still work on integration after republishing this route.