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/coredata/migrations/20260529T617403Z.sql

@@ -1,15 +0,0 @@
--- Copyright (c) 2026 Probo Inc <hello@getprobo.com>.
---
--- Permission to use, copy, modify, and/or distribute this software for any
--- purpose with or without fee is hereby granted, provided that the above
--- copyright notice and this permission notice appear in all copies.
---
--- THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
--- REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
--- AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
--- INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
--- LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
--- OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
--- PERFORMANCE OF THIS SOFTWARE.
-
-ALTER TYPE connector_provider ADD VALUE IF NOT EXISTS 'POSTHOG_SELF_HOSTED';