feat: run all migrations in a transaction (#10966)

Updates coder/customers#365

This PR updates our migration framework to run all migrations in a single transaction. This is the same behavior we had in v1 and ensures that failed migrations don't bring the whole deployment down. If a migration fails now, it will automatically be rolled back to the previous version, allowing the deployment to continue functioning.

coderd/database/migrations/000114_workspace_proxy.up.sql

@@ -1,4 +1,3 @@
-BEGIN;
 CREATE TABLE workspace_proxies (
     id uuid NOT NULL,
     name text NOT NULL,
@@ -19,5 +18,3 @@ COMMENT ON COLUMN workspace_proxies.wildcard_hostname IS 'Hostname with the wild
 
 -- Enforces no active proxies have the same name.
 CREATE UNIQUE INDEX ON workspace_proxies (name) WHERE deleted = FALSE;
-
-COMMIT;