Deprecate non-ClickHouse / Postgres destination connectors

[mshustov]

Context

We are deprecating PeerDB's non-ClickHouse / non-Postgres destination connectors.
They are no longer actively maintained.
The Postgres → ClickHouse, Postgres → ClickHouse Cloud, and Postgres → Postgres paths remain actively maintained.

Deprecated connectors stay fully functional in current and prior releases.

Scope

Destination connectors to deprecate: Snowflake, BigQuery, ElasticSearch, Kafka, Redpanda, Azure Event Hubs, Google PubSub.

No code is removed as part of this work.

Tasks

Public copy

• Add a deprecation notice + connector status matrix (Actively maintained vs. Deprecated) to the README
• Add a per-connector migration note (how to pin to the current version + fork instructions), linked from the README and auto-responses.
• Add a Deprecated badge to these destination in the UI creation flow

GitHub automation

• Add a "destination connector" field to the issue template that surfaces the deprecation notice inline when one of these connectors is selected.
• Add a workflow that posts a standard comment + applies a deprecated label when a deprecated-connector label is set, or when a PR touches the relevant flow/connectors/{...}/** paths

Community

• Apply the deprecated label + standard comment to the other currently open non-CH destination issues/PRs. Do not auto-close them.
• Migration notes in CONTRIBUTING for deprecated connectors: pin + fork

[Jeremyyang920]

PeerDB-io/docs#246