Merge PostHog self-hosted into a single PostHog provider

Fold POSTHOG_SELF_HOSTED into POSTHOG: one provider now covers Cloud (OAuth + region-pinned API key) and self-hosted (API key + instance URL), since both already share the driver, name resolver, and PostHogConnectorSettings{BaseURL}. The API-key form picks a deployment (Cloud US/EU or self-hosted URL); the resolver requires exactly one of region/instanceUrl.

Drop the POSTHOG_SELF_HOSTED enum value, registration, migration, and logo mapping. Extract the deployment selector into a dedicated PostHogDeploymentField component. Point the driver tests at us.posthog.com instead of the legacy app.posthog.com host.

Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>

pkg/accessreview/drivers/testdata/posthog.yaml

@@ -7,7 +7,7 @@ interactions:
         proto_major: 1
         proto_minor: 1
         content_length: 0
-        host: app.posthog.com
+        host: us.posthog.com
         form:
             limit:
                 - "100"
@@ -16,7 +16,7 @@ interactions:
         headers:
             Accept:
                 - application/json
-        url: https://app.posthog.com/api/organizations/@current/members/?limit=100&order=-joined_at
+        url: https://us.posthog.com/api/organizations/@current/members/?limit=100&order=-joined_at
         method: GET
       response:
         proto: HTTP/2.0